Wednesday, February 8, 2023


Tips to clean a gunky gray water tank. Yes, yours is gunky too

This all started when our suction cup soap holder in the shower fell off the wall. No big deal – it falls off every couple of months when jostled too much or the suction cup dries up. I decided to clean the soap residue from the soap holder and while at it I cleaned another soap container. The soap didn’t just rinse off and both items needed a good soak.

The bucket I was soaking them in ended up full of soap scum and I realized that the residue in the bucket was a microcosm of what must be in our gray water holding tank. It was time to clean.

Prevent “stuff” from getting into the tank

First prevent the stuff that sticks, clogs, and stinks from getting into the gray water tank. Start at the source.

  • Use a kitchen sink strainer and a hair strainer in the shower. We use this one in the shower after getting a long hair clog (mine) and it works great! Sizes vary, so double-check the drain size if ordering.
  • Scrape and wipe off dishes well before washing.
  • If using an RV dishwasher, be aware it is not connected to a garbage disposal like home dishwashers. The dishes need to be scraped and food particles wiped off too.
  • Clean your sink strainer by lifting and wiping down periodically. Pick out any scraps that have made their way into the drain.

Add cleaner to the gray tank

Add cleaning agent to the tank. Do NOT use bleach. Bleach is hard on rubber and can damage valves and seals. There are a number of recommended cleaners. While any of these will work, Dawn dish detergent is the most popular – both because of its grease-cutting ability and being readily available.

Recommended cleaning agents

Choose one (only one!) of the suggestions below.

  • 1/2 -1 cup Dawn dish detergent is one of the most popular
  • 1/2 cup Calgon water softener and 1/2 cup dishwashing detergent
  • 1 cup dishwasher gel
  • 1 cup baking soda
  • 1 cup laundry detergent

Fill the gray tank with water and drive around

Fill the tank with water. Drive your RV around and let the cleaner slosh around and break down the gunk. Let the cleaner stay in the tank for a day or two if possible. This is a good thing to do when traveling and stopping at another full-hookup site.

Dump tank and rinse

It is good to rinse the tank at this point if you have a tank rinser or sewer hose rinse adapter. You can get the adapter here. This is a 45-degree one but there is also a straight flush attachment available.

Add treatment

You can add 1/2 cup to 1 cup dish detergent to help keep the tank clean after dumping or any enzyme-based gray water treatment.

Some people add a treatment every time after dumping and others only the 2nd or 3rd dump cycle.


RV Education 101: RV gray water tank maintenance tip



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6 months ago

Hi, we’re getting a camper for first time. 30+ yrs tent campers. Please forgive what to most will sound like “duh” questions. Rig has 20g fresh water tank and 22g gray tank, cassette toilet. So only one sink and one shower drain (shower we may not use right away.) I’m good with the cassette toilet = filling/emptying/cleaning.

For gray tank, do you empty it first then put the Dawn, Borax, water mixture down the sink drain and let it run into the gray tank and even let sink faucet add more water to the tank so there’s enough in there? Then do you let that slosh as you’re driving home or to next campground? Arriving at next campground, say 8-10 hrs later, do you stop first at dump station to empty that fluid out? Must it then be rinsed: I wouldn’t think the hose at dump station would be right, cause black tanks get rinsed out with those hoses.

Like I said, this is new territory for us and I need to visualize the process.

Thanks very much for help!

RV Staff
6 months ago
Reply to  Deb

Hi, Deb. Welcome to RVing! For answers to your questions about the holding tanks, I suggest you go to then search either for “holding tanks” or “black tank” or “gray tank”. Any of those options will bring up dozens of articles, and I’m sure you’ll find lots of useful information. We hope you have many happy years of RVing! 😀 –Diane

1 year ago

I have read many of the comments and agree with most, but campers are different. I think there needs to be advice given to many campers which is “Water is your friend!” Some toilets do not use enough water when flushing. We also need to advise people to add more water when draining tanks and to add water and drain, add water and drain again!

1 year ago

We’re full time, and are careful about what we put down the gray tank, but it still wouldn’t drop below 1/3 or 2/3. I spent hundreds of dollars on commercial and home treatments. After 2+ years, I recently tried a gallon of white vinegar in a tank about 1/3 full before a travel day. Dumped about 36 hours later and it has been going to E ever since. Something to try.

Barry T
1 year ago

I agree with Lance Craig. I am also amazed at the constant obsession for holding tanks. The tanks are there for a purpose…to be used. Yes, a good flushing out periodically is a very good idea. I am even more amazed at those who refuse to use their onboard toilets!! Even more and more amazed at those who throw used toilet paper in a bucket for later disposal. All these ideas seem to have gotten more prevalent in the last few years.

It’s a toilet!!! It’s designed to be used. With a little care, all will be well. We full-timed for 10 years and never had a problem with our holding tanks and only did occasional flushing with water. Come to think of it, our full-timing motorhome did not have an external valve to attach a hose for flushing. When at a campsite I would fill the tanks, add some detergent, and empty the next day.

1 year ago
Reply to  Barry T

I agree with Barry. On board gray and black tanks are designed to be used and throwing used TP in the garbage is unsanitary. A great way is to keep systems working is to drain tanks, add a cleaner and 10 gals of water, then drive to the next site and dump/flush tanks. I think one of the biggest issues with tanks is not using enough water when flushing toilets and emptying. Remember this – WATER IS YOUR FRIEND. Use lots of water when you use the bathroom and flush tanks when you can.

Stan W
1 year ago

I read an article about soap scum on the shower walls at home. It said that if you want to get rid of soap scum the using soap. It said use body wash, they aren’t soap, they are detergent. I have tried all the mixtures to clean out the waste tanks, I don’t think they do much other then waste your time and money. For last 2 years for the black tank i use the stuff that advertised here, Caravan, its an enzyme or something like that. I follow their directions and it works great, no odors. Also when I flush the tank after the 2nd flush not much comes out. Gray tank I get a lemon deodorizer, also recommended here, use that every time I start noticing a smell.

1 year ago

Be aware that driving around has been proven to do nothing to help the cleaning process.

1 year ago
Reply to  Drew

And if the tank is full, there will be no “sloshing around”.
Just another solution looking for a problem.

1 year ago
Reply to  Drew

James of proved adding ice cubes and driving violently didn’t clean a test tank filled with NASA approved artificial poo.

1 year ago
Reply to  Drew

“Proven”? By who, and under what conditions? The “Ice cube” test has been proven, but I have never seen anything to indicate that the sloshing of a half-full tank with some added detergent or other product will “do nothing to help”. In fact, I have seen the opposite. When my sensors get a little dirty and don’t read correctly, the half-fill sloshing tank, with a cup or two of Dawn cleans them right up. A couple of hundred miles on most roads will do the trick for me.

1 year ago
Reply to  Tom

I agree with a 1/4 – 1/2 half tank ( I think 1/4 full would be better) or 1/8 full water and 20 lbs. of ice with 1/4-1/2 cup of Dawn added immediately before driving will clean the tank. But this is not a “One and done” process. If possible drain the tanks, add the water (ice cubes if you want) and cleaner and drive to your next campground or home (you can dump at Pilot’s and many rest areas if needed). When you reach your destination, dump as soon as possible and flush well. Keep in mind, you need to do this regularly to keep tanks and sensors clean.

1 year ago

We do nothing to clean the grey tank (though we do use the grey water to sluice out the black tank, using an added 3″ valve at the discharge end). After 12 years we figured we might have a mess in the grey tank, so hired one of those mobile tank cleaning outfits who can power-wash the inside of the tanks. He reported there was next to nothing in either of our tanks. So I’m thinking all of this worry is being wasted for most of us…

Lance Craig
1 year ago
Reply to  Don

I agree. All this worry about cleaning a waste tank, black or gray, is amazing to me. I do use the black tank rinse to clear any lumps left on the tank bottom, but that is it. No chemicals in either tank. No effort spent “cleaning” a waste tank. Like it is going to be “clean” more than a few hours at most. We have no odors, maybe due to 360 siphon vent caps. Always keep a little water in the tanks if the coach is sitting for a while unused so things don’t dry out and stick to the bottoms. (Note: When we were new to this, we used every kind of tank treatment we could find to see what worked best. As time went by, I would forget to add treatment until it occurred to me that it didn’t matter as long as the coach was used at least monthly. Granted, if you only use an rv infrequently, getting tanks relatively clean before storage may be more important.)

Philip Sponable
1 year ago
Reply to  Don

Yep… same here on my 2003 DP… ‘next to nothing in either of our tanks’… but we are a low user.

1 year ago

Somewhere while reading all the tips to clean your gray and black water tanks I came across this solution;
1/2 cup Dawn Dishwashing liquid
1/2 cup BORAX laundry detergent enhancer
2 1/2 gallons of water mix well and divide between your two tanks. The bigger portion, in your black tank.
You can do this every time you drain your tanks or at whatever interval you choose.
It works great!
Happy RVing and hope to see you down the road!

Jesse W Crouse
1 year ago
Reply to  Boltman

I use the same mix and since doing this NO odor. All those years wasting money on colored tablets never stopped the odor. Two years and only the sweet smell of success.

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