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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RV StaffDick RohdeMike SchwabMatt Scott Recent comment authors

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Dick Rohde
Dick Rohde

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… Read more »

Mike Schwab
Mike Schwab

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
Matt Scott

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… Read more »