Monday, September 25, 2023


Really? Does your RV’s bathroom sink drain into the sewer tank?

Most RVers assume the sinks in their RV drain exclusively into a gray holding tank. The toilet, most RVers believe, drains exclusively into the black (sewer) tank. But is that always true? In this eight-minute interview by editor Chuck Woodbury with RV waste systems authority Doug Swarts of, you’ll learn that sometimes the bathroom sink does, in fact, drain into the black tank.

Doug even points out that at least one manufacturer drains the liquids from the sink, shower and toilet into the same tank!

While it may be no big deal to most RVers where the water in their bathroom sink drains, to some it can, indeed, be a big deal when conservation of liquid waste is important — as in boondocking, when every ounce of waste liquid matters.

After you watch the video, please answer our brief poll directly below the video. Do you know where the water in your bathroom sink drains?

Okay, would you now please take our survey. Thanks.



Chuck Woodbury
Chuck Woodbury
I'm the founder and publisher of I've been a writer and publisher for most of my adult life, and spent a total of at least a half-dozen years of that time traveling the USA and Canada in a motorhome.


  1. I have a 1987 Toyota Sunrader. The shower and toilet are in the same space. The toilet sits on the drain pan (designed that way.) The toilet goes straight to the tank and the shower pan has a drain trap to the tank. There is no room to cross over to the Grey water side of the vehicle.

  2. The first simple check I will perform when I get my RV in Oct is to open the black and grey water tank and take a hose inside the unit and run water into each sink, toilet, and shower drain until the water runs out! That way I can confirm which drain runs to which holding tank! For used units, You can do the same after dumping and flushing both holding tanks.
    Happy Camping!

  3. My first tt drained the bathroom sink into the black tank. I was told that was to insure there was plenty of water for the black tank to function properly. It was a 24 1/2’ 1978 CampMate by Tech.

  4. In the late 80’s, when I ordered a new fifth wheel, manufactured in Washington, the dealer told me that the bathroom sink drained into the black tank to make sure it would have enough liquids, also that there were a couple states that it was a law that the black tank needed an additional drain source other then just one.

  5. Yep, my Thor Chateu bathroom sink drains into the black tank.
    Just this morning, I had to pull the check valve on the sink drain pipe since it seemed to be stuck open and the RV smelled like a sewer.
    I found the valve full of maggots! Ugh!
    Dam* flies.

  6. I sold my 2011 Damon Daybreak 27PD, last December. I discovered, early that my shower emptied into the black tank. Whenever the black got near dump time, I would run the shower till it got 2 or 3 inches of water. Then would drain black tank, and rinse shower.

  7. We have a 2003 Dutchman Express 26Q. I found out pretty quickly that the bathroom sink drains to the black tank. Since the grey water tank is only 20 gallons (probably the smallest in the industry) and the black is 25 I guess it’s not such a bad thing.

  8. I have a 2014 Winnebago Adventurer. I know that my sinks drain into the gray tank and the dealer told me the washing machine also drained to the gray tank. Imagine my surprise when the black tank filed up when doing our first load of clothes.

  9. We just bought a ’71 Ozark Luxury Liner – it has no bathroom sink. The kitchen sink and toilet drain into an exterior portable black tank. The shower drains onto the ground. It looks like originally there was a single black tank on the underside for all three (I can see where the straps were). We may keep the exterior black tank, but we’ll be redoing the entire drain system.

  10. Draining to the black tank can result in the SPRAYING OF SEWAGE when you open the toilet drain valve…

    When the black tank gets over-full (because say you’ve only used the bathroom sink and not the toilet), and then the sink drain backs up, then the PRESSURE upward on the toilet drain valve is POSITIVE (about 3 ft of head). This will result in a surprise sewage spray when you open the toilet drain valve, or at least the possibility of sewage leaking around the toilet valve and into the bowl.

    I just discovered this draining into the black tank on my ’94 Fleetwood Southwind when, while doing maintenance on the toilet and washing my hands in the sink, I HEARD it draining into the black tank. Otherwise, I would have had no clue.

  11. Easy to find out. With all tanks empty. Open the black water drain. Turn on faucets and shower see if any water comes out black water drain.

  12. I was mystified by how fast my black tank filled up while parked for the winter at a FL RV park. I was dumping it about every 4 days. No way were we using the toilet that much. A neighbor suggested that perhaps another fixture, such as the bathroom sink, was draining into the tank. So I performed a simple test.

    • Simple test (continued): drain both black & grey tanks & flush. Run substantial quantity of water down the drain of suspect fixture. Open black tank dump valve. Any water discharged? Open grey water dump valve. Any water discharged? You will easily determine where each fixture drains to. What did I find? The kitchen sink drained into the black tank! That’s why we were filling the black tank so quickly! Now I’m trying to determine if and how I might replumb the drain. Doesn’t look good.

  13. I have a 1955 Spartan Mansion (32 ft) trailer that was originally a park model with no tanks. The toilet sits between the two axles with minimal clearance so the best design I could come up with is a 26 gallon black tank for my toilet, kitchen and bathroom sink connected to a 12v macerator pump (which pumps to another 26gallon auxiliary tank, then to my sewer connection. My shower is on a separate 26 gallon gray tank with a separate sewer connection so in total I have 72 gallons of gray/black storage. I bought the tanks from Elkhart Plastics which was the only tank supplier that had a tank with the dimensions i needed.

    It works great but I dread the day the macerator pump breaks. Macerator pumps are used in boats all the time so they do have a good track record.

  14. The manual says the bathroom sink drains into the black tank. I know this is right because you can smell the fumes coming out of the sink when water is running or when we are driving. Thought I’d put in one of those white things you see advertised that take out the u-joint and says it stops the stink. Turns out there is already one there. I now leave the plug in all the time with a cup of water in the sink. No more stink. Have to be careful brushing teeth not to get a mouthful of stinky air when spitting.

  15. We have a 2017 Redwood and while just getting it back from the factory shop (4th time) we wanted to be sure that they fixed ALL the tank level indicators. With empty tanks we first stuck a garden hose into the kitchen sink and filled it until it was backing up in the sink…ok. that one’s full….however the indicator showed 1/3 filled. Then to the shower…same thing, filled till I had water above the drain (indicatorshows 2/3d full)…well, might as well flush the bathroom sink a little to clean the gooseneck… my surprise the water level in the shower didn’t come up…? but we checked the black tank and showed enough water in it to light up the black tank indicator. Finished by filling the black tank with the hose. my thought is that the drain for the washer (prep) was going into the shower gray tank instead of the black. The tanks all needed fresh cleaning anyway…found out I can take longer showers now!☆

  16. I have a Heartland Edge M18. I haven’t confirmed but suspect the sink drains into the black tank based on how quickly the black fills.

  17. Something related but not mentioned: I’ve seen standard RV “drop” toilets (as opposed to macerating toilets) where there is a SHARP BEND (think ~ 90º) IMMEDIATELY and, the “solid waste” MUST make said sharp bend before it is carried off to the black tank. In stick & brick homes, waste lines MUST be CAREFULLY installed. I believe the the requirement is 1/8″ drop for each 12″ of pipe run. That’s a VERY small margin. More than 1/8″ drop allows liquid to drain faster than it can carry solid waste away. Less then 1/8″ drop allows the solids to collect IN the pipe rather than be carried away. This means that LEVELING the RV would be critical (even more so than an RV refrigerator requires). Fail to achieve this high level of accuracy when leveling an RV virtually guarantees plumbing problems. I saw this type of drop toilet at an RV rally. I have no idea if this installation technique is commonplace. RVers with this plumbing should probably use extra water when flushing solid waste.

  18. My 84 Toyota Dolphin front bath has sink and shower draining into black tank-found out hard way. Ugh. After much research on replumbing etc I switched to a C-Head compost toilet and now have 2 grey tanks and no black tank. All working very well!

  19. We discovered the bathroom sink drains to the black tank shortly after we purchased our Class C motorhome. I found a deep plastic kitchen food storage container that fits perfectly in the sink, which we use for tooth brushing, shaving, etc. Then it is dumped down one side of the kitchen sink, and into the grey tank. Helps keep the black tank from filling so quickly.

  20. A simple way to check where your sink(s) empty into would be to empty ALL holding tanks. Then, empty several gallons of water down the sink you want to test. Open the BLACK tank and check if water is running through the sewer hose. If water IS running through said hose, you know that sink empties into the black water tank. No water emptying through said hose? Then close the the black tank valve and open the grey tank valve. If water IS emptying through the sewer hose, that sink empties into a grey water tank. The easy way to run this test would be at a FHU (Full Hook Up) RV site. The tests shouldn’t take very long and, you’d know for sure which sinks / drains on YOUR RV empty into which tanks. It would also be a good time to give your holding tanks a thorough cleaning.

  21. Why do we really need to separate gray and black waste? Why not just one “integrated” tank, or more for larger rigs? I could understand the need in the distant past when we drained our gray water onto the ground, but now?

    • Not that long ago I was at a park that upon check in I was told I could dump gray on the ground. No site hook-up but had a dump station. I assumed it was so they did not have to pump the dump station as often.

  22. Chuck,
    Next spring on my first camping trip, I’m going to fill my bathroom sink & plug it with the stopper. Then I’ll turn off the pump & flush the toilet until the water stops running in the toilet. Then with the toilet valve still open, I’ll empty the sink. I should be able to hear or see the sink water splashing into the empty black tank, if that’s where it goes. Then I’ll do the same thing with the tub.
    What do you think?

  23. Our puma 295bhss has two grey tanks. Aft tank only is fed by the outside kitchen sink. Forward grey tank is fed by the main kitchen sink, bathroom sink, and the shower. It fills up fast unless crosslinked with the aft tank (giving full 70 gallons of capacity). Got one of those external gate valves to link the two tanks without making a total mess when uncapping the sewer cap.

  24. Scamp 16 trailer. No bathroom sink. Bathroom shower pumps to grey water tank. Kitchen sink (only sink) gravity drains to grey water tank.

  25. I’ve had three motorhomes , a Triple E, an Empress and a Miramar. All three bathroom basins have drained into the black tank. It’s very easy to tell which tank the basin drains into. Have your tanks empty, with the tank valves closed and fill your basin. Drain the basin, open your grey tank. If water comes out it is going in the grey tank, if not it is going in the black tank.

  26. Learned that the bathroom sink drained into the black tank while cross country traveling. We were definitely surprised. Didn’t like the black tank filling up so quickly, so we started thinking of “work arounds”. Didn’t like the thought of washing our hands in the kitchen sink, so, if we are traveling or boondocking, we place bowl in the bathroom sink. Then, when full, dump it in the shower drain. I was even thinking about rerouting the plumbing.. but that is for another day.

    • I dont understand why your worried about washing your hands in the kitchen sink? Our RV bathroom sink has a standard trap under the sink which protects any smells coming up thru the black tank into the sink… just like houses are built. If your RV doesnt have a trap then that is a big problem and makes no sense.

      • You are so correct. I know it doesn’t make actual sense, it’s just the thought of using the facilities, then washing our hands in the same place we wash our vegetables, prepare meals and fill water bottles.

  27. My dad bought a 19′ Open Road motorhome in early 1973. We (kids, me just 21) didn’t pay any attention to the details of plumbing until one day my kid sister went to use the bathroom & let out a shriek. We’d gone one day too many before emptying *the* holding tank. Apparently the water tank was either bigger than the holding tank or close enough that the holding tank needed to be emptied before the water tank ran out. A lesson I’ve carried with me ever since.

  28. In Jackson Hole, when sitting in cold weather, let bathroom sink drip and found out this drained into black tank. Just mint that you had to drain black tank more often and it drained out nicely. Always left gray water tank open.

  29. I answered “Other” because our camping trailer doesn’t have a bathroom sink. However, the shower drains into the grey tank.

  30. How about this: I have the Montana FB3921 with a tub in the master bathroom. With “skeight-eight” gallons full, where does that drain?!

  31. I’ve only driven small class Cs and luxury Bs, so I assumed (correctly) that my bathroom sink drained into my black tank. Since the black and gray tanks in many small motor homes are close to the same size, it’s actually good to have the bathroom sink draining into the black tank, as it gives you more time between dump station visits. Also, all the soapy water makes dumping the black tank easier.

  32. We have a 2003 26′ Dutchman Express which we purchased new. Found out by listening that the bathroom sink definitely drains in to the black tank. The dealer said it was probably designed to put soapy water in there. Pretty sure it was a cost cutting design. On the other hand this camper has one of the smallest grey tanks in the industry at 20 gallons so I guess it is to our advantage.

  33. It’s bad news if your shower drains into the black tank since it’s the lowest drain. Often we know our gray tank is full because it backs up into the shower. The gray water is gross enough, I can’t imagine black water in the shower.

  34. On our Leisure Travel Van I have learned (I think this is correct) that the bathroom sink drains into the black tank. At first I thought this was not such a good thing but on second thought, with our previous motorhome we would always fill the grey water way before the black tank. Many times we had to dump the grey but the black was only 1/4 full. So in retrospect, having some of the grey water going into the black may not be such a bad thing after all and for the small amount the bathroom sink is used it really doesn’t make allot of difference for us.

  35. My Thor Hurricane Master bath drains to the black tank. No one told us. We found out when we left it running to not freeze the pipes.

  36. I know that my shower drains into my grey tank. The reason I know that is that my grey tank is the fastest to fill. And because I can’t imagine a floor plan layout that has the bathroom sink very far from the shower, I feel fairly sure that most bathroom sinks also drain into the grey tank, but I don’t know for certain. It wouldn’t be very hard to find out; simply turn on the water in the bathroom sink and let it run while occasionally checking the tank gauges until the gray or the black gauge registers an increase. Or, if the grey tank is empty, open the grey tank drain valve, turn on the sink water, and see if it comes out the tank drain. If not, it must be going into the black tank.

  37. Can you provide a transcript for these types of articles? I can read about it a lot faster than watching a video and it saves bandwidth.


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