Monday, March 20, 2023


Can black tank left open for years on seasonal RV be cleaned?

By Chris Dougherty

Editor’s note: Chris Dougherty is a certified RV technician. Here is a letter he received from a reader while he was serving as’s technical editor.

wonderingDear Chris, 
I just purchased a 2010 36-foot Forest River Salem FLKB camp trailer. It’s been on the same site since it was purchased new, and the black and gray water tanks have been in the open position. From everything I am reading this a big NO-NO. I have purchased the necessary chemicals, I think, to start treating the tank properly. My question is, will this be enough to drain the tanks? It shows by the indicator lights that it is 2/3 full. Am I in for a big expense to clean it out? The trailer was VERY LIGHTLY used before I purchased it. In the past four years, I would say the previous owner used it perhaps six to ten weekends. —Chris

Dear Chris,
All is not lost, and it won’t cost you a fortune, hopefully. The key is to flush the tank out completely. I would fill the tank with water after adding at least a double dose of chemical like Odorlos or PurePower Blue. Let it sit for a few days while you’re gone from the coach. Get yourself a Camco Swivel Stik tank rinser and after the tank has emptied use this to flush the tank. Another device which can help if the black tank termination goes straight up into the black tank with no elbows is the Camco dual flush rinser which will shoot a jet up into the tank.

With it being a permanent, seasonal unit, there’s not a lot more you can do to clean the sensors. If this doesn’t work, there are aftermarket sensors with guards that prevent false readings, according to the manufacturer. They are made by Horst Dynamic and our friend Gary Bunzer, the RV Doctor, has recommended them.

Editor: Here are links to holding tank chemicals and Camco tank rinsers at Amazon.


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Sally G
3 years ago

Friends of ours bought a used 5th wheel that was in wonderful condition – except for the black tank, which filled really quickly and was constantly smelly, despite all their attempts at cleaning and clearing it. They eventually had it professionally cleaned, and it was amazing how much crud came out. It’s perfect now.
They figure the black tank had always been left open, by the previous owners, causing solid waste to harden in the bottom of the tank. They couldn’t use a wand to get into the tank due to the configuration, whereas the professional had all the right equipment to access it. Didn’t cost a fortune, as far as I recall, but still unpleasant to have to get it done, simply because the previous owners hadn’t understood how to look after the holding tanks.

3 years ago

Somehow, figure out how you can fill them tanks up with hot water. I am guessing the trailer isn’t in your driveway where you have access to the hot water tank at your house. Don’t even bother with chemicals will be wasting your money.
You will have to do this a few times. Let sit over night each time. Will clean your sensors as well as your tanks.

David McMullin
3 years ago

To add a little more on the RV black tank cleaning, multiple full tank flush outs will help a lot.
If you are a diy person and have access to a presser washer you can buy a drain cleaning jet for it and this will clean out your black tank like us professionals do it.
Safe Travels.

Sharon B
3 years ago

I have a question about the black water tank. I am in the process of changing to a compost toilet. This will enable me to have more fresh water. I don’t believe it is possible to really professionally disinfect the black tank so I assume it will need to be changed completely. I would also disinfect a new tank too. I have not gone under the trailer yet to see how it is secured so I am prematurely asking questions.
Has anyone done this and if so have any recommendations.
I also realize the gray water cannot be let out in any BLM’s or other campgrounds. My gray water is only from the shower or kitchen sink. I never use grease but only wash fruits/ veggies or wash dishes.

Ed Kilburn
3 years ago

If cleaning the tanks doesn’t work as described. And you have a pressure washer, purchase a Clog Hog from I use this that I drilled out to fit the Cloghog tip in. Rubber gloves and you are all set, no mess but can get stinky. Run the clog hog in with one tank open at a time so you know where you are. I find leaving the water flow without having the washer on makes it easier to get the nozzle into the tank.

Dick Rohde
6 years ago

There is another solution that works if the valve can be closed. Pour an ample amount of concentrated cleaner/degreaser such as Simple Green into the tank. Add about a half tank of cold water and 4-5 bags of ice. Drive over a rough road that will swish the cleaning solution around. This is an old trick that works and is inexpensive. You need to be ready to move as soon as you put the ice in for obvious reasons. A good way of doing this is when traveling between campsites. You may want to stop along the way and add more cube ice to provide more scrubbing.

Bob p
3 years ago
Reply to  Dick Rohde

The problem with ice is that it floats on the liquid and does not touch anything but the sides. There are some videos on you tube that show this situation so what ever is stuck on the bottom will not be scrubbed. Also the ice you buy from an ice machine doesn’t have sharp edges so basically you are putting cold non melting marbles in the tank, and if you’ve ever put ice into a glass with any kind of liquid that is the temperature of tap water it doesn’t take but a few minutes for it to melt. Now if you were to put enough ice in to cover the bottom of the tank with out water and took it for a drive it might scour it some before it melted. Good luck.

Mike Schwab
6 years ago

Instead of chemical, use the Geo Method. Add 1 cup Blue Dawn dish soap, 1 cup Borax or Calgon, fill black and grey tank with water. Let sit a two or more days. Drain.

After every dump, add 1 cup Dawn and 1 cup Borax to Black Tank with a gallon or two of water. If tank indicators read empty, you can skip on this use.

Matt Scott
6 years ago

Folks, I would NOT recommend the use of a tank rinser that is inserted through the toilet. What happens is that the water spray will produce a mist that will mix with contents of the tank and float up into the room. You will immediately be breathing that mist and it will be settling over all surfaces of the bathroom. Not a good plan.
The problem I have seen with blackwater tanks that have been left open is that the valve will not close properly. If that is the case, the first step will be to replace the valve. Then you start with the flushing process.

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