Tuesday, November 28, 2023


Gray water tank can smell more than black. Here’s why and what to do about it

Who knew I could get so excited about flushing the gray and black water tanks? Okay, I know it is a bit odd, but as the designated tank flusher I am happy when the “gunk” comes out. While I am diligent about flushing out the black water with the built-in flush system, there was no built-in system for the gray water. When I complained to our RV tech about the smell when driving, he said that the gray water can smell worse than the black.


Mold in the gray water tank

I recently attended a rally seminar about “All things black and gray” and, boy, did I learn a lot! The presenter from Kleen Tank passed around glass bottles of “stuff” that had come out of holding tanks. He did advise, “Don’t open, don’t drop,” while handing it off.

I recognized the black discs that had been coming out of the gray water holding tank and was disgusted when he said it was a massive amount of mold. He also had some discs as hard as metal that he passed around that had “mineralized” with the urine in the black tanks. He said that he finds the gray tank is usually worse than the black, particularly if people have built-in flush systems and had been keeping up with the black tank.

Flush rinser with gate valve and water meter

Best of all, he passed around this flush attachment and water meter. I knew the flush rinser existed, but our hose goes straight down through the bay and there was no way the flush attachment was going to fit. But wait! He showed how the gray and black dump pipe rotates outward and allows the flush rinser to attach! This is what he showed me.

Combined with a water meter, the flush rinser can easily clean out both gray and black water tanks.

The water meter measures how much water is going through a garden hose. It is usually used when watering a garden but it is an exceptional tool for measuring how much water in gallons or liters is going into RV tanks, particularly when flushing. (You can read more about this handy water meter here.)

Warning and Disclaimer:


It is not recommended because there can be disastrous results if one walks away from the RV or gets distracted while filling a closed gray water tank or closed black tank with water. Think black water eruption through the toilet into bathroom…

With that disclaimer noted, this was the method taught at the seminar to thoroughly flush out the black water and gray water tanks. This was also the method Kleen Tank used when flushing out our tanks. They had the additional step of a hose inserted into the tanks to pressure wash them along with flushing them out. That pressure wash is what a professional is for!

Flush steps

  1. IMPORTANT! Double-check the gallon capacity of the holding tanks in the RV specs. If you are like me, it is a good idea to write them down!
  2. Attach water meter to a garden hose. Do NOT ever use a fresh water hose to flush! Use a backflow preventer to prevent contaminated water going into fresh water faucet. Kleen Tank adds the backflow preventer and water meter at the flush rinser. Others put the preventer and meter at the faucet.
  3. Attach the flush valve rinser to the sewer pipe opening and sewer hose.
  4. Pull gray water valve open for a few seconds and close to “lubricate” the sewer hose.
  5. Dump black water.
  6. When black tank is empty, follow directions on the flush valve rinser, close the rinser gate valve and fill the tank with water.
  7. Watching the water meter, fill UNDER the capacity of the tank. I went under by 20 gallons just to be safe.
  8. Turn off water and pull gate valve. If you can stand to watch, see how much more gunk was in there.
  9. Repeat with the gray water tank. If your tank was like ours, it was not a pretty sight!
  10. This flush rinser has a hose rinse setting too, but I found it rather ineffective and figured the gray water going through the sewer hose was rinse enough.
  11. Add at least two gallons of water to the black tank through the toilet and then treat as normal. A number of people are so happy with this type of flush they don’t add any chemicals, organic or otherwise, to the tanks.

And that’s what you call a royal flush! Happy flushing!


Nanci Dixon
Nanci Dixon
Nanci Dixon has been a full-time RVer living “The Dream” for the last six years and an avid RVer for decades more! She works and travels across the country in a 40’ motorhome with her husband. Having been a professional food photographer for many years, she enjoys snapping photos of food, landscapes and an occasional person. They winter in Arizona and love boondocking in the desert. They also enjoy work camping in a regional park. Most of all, she loves to travel.



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Neal Davis (@guest_243927)
4 months ago

Thank you, Nanci!

Dale M (@guest_241947)
5 months ago

Why not install a flusher on your gray water tank. There are kits for this, Camco 40123 Quickie Flush with Back Flow Preventer. I have done it on my last two RVs and no problems.

Thomas D (@guest_241908)
5 months ago

I’m thinking this only fills the gray tank. no pressure washing or even movement to the sidewalls. If correct, why not just fill the sinks and dump the drains. I use rid-x at home for my sceptic tank and in the rv (mini septic tank).

Bob P (@guest_241902)
5 months ago

You must not rinse your plates with food scraps down the drain, these food scraps will sit on the bottom of the tank and decay, that’s where the rotten odor comes from. In my 40+ years of camping we never rinsed our dishes with food scraps down the drain. It’s a common practice at home and the scraps go directly into the sewer or septic tank, neither of which you have in a RV. Scrape the food scraps into the garbage can, then you can wash the dishes. This was a no-no tip my mother passed to me back in 1978, nothing but dirty water goes in the gray tank.

Joe (@guest_253031)
2 months ago
Reply to  Bob P

We purchased screens at a box lumber store specifically made to go into the kitchen sink to catch 99% of all food scrapes. They are that fine of a screen that coffee grounds do not pass. Between the screens, Dawn dish soap, and some Rid-X I have never had an issue.

Jim Johnson (@guest_241888)
5 months ago

I’ve been using a flow meter / back flow preventer combo for a couple years now. Prior to finding out about inexpensive flow meters I used to guestimate how many gallons per minute the campground was supplying and stare at my watch.

The above link to a two pack has been a good buy. I use one for waste tanks and one to measure the input into the fresh water tank. I was a little skeptical about the accuracy so I used an empty translucent one gallon milk jug with variable flow rates. When the meter said one gallon, the jug was filled to exactly the same level as the purchased milk. FAR more accurate than using wonky tank sensors and you don’t have to run forth and back.

mark cieslikowski (@guest_241884)
5 months ago

How are you supposed to fill gray tank through the outlet hiose when you have a pump connected to the drain system

TIM (@guest_241881)
5 months ago

It doesn’t matter how bad the liquid in any waste tank smells, if you are able to smell it inside the RV you have a plumbing problem that needs fixed. Traps, air admittance valves, and vents are all designed to prevent odor inside the RV.

Tom (@guest_241876)
5 months ago

I find that a jug of cleaning vinegar in each tank helps a lot. Mold will not grow in the vinegar environment. Dollar Tree is a inexpensive source.

Diane Tricomi (@guest_241882)
5 months ago
Reply to  Tom

i would like to know if you just pour the vinegar down the drain ? or mix with water?

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