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The black tank of doom and how to “cure” it.

Within the RV community, inaccurate black water holding tank sensors are known for being one of the most frustrating issues anyone can run into! It can sometimes seem like you’ve tried everything possible to restore your sensors, and they still won’t read correctly.



For example, many peoples’ sensors will read “full” when the black tank is only a quarter filled! Here is a great question we often get asked:



"So, we are about a week into moving in our used Grand Design Imagine. On our way home with the trailer, we stopped at a RV park and emptied the tanks (black tank was 2/3 full upon purchase). I never checked the gauges inside, I just visually looked for liquid to stop flowing. Now I've gone to empty everything again but can't seem to get the black tank below 2/3 full. We've only used the toilet for #1 since we've owned it, even throwing away our TP in the trash. Are there any possible DIY options to fix this, or is it best to take it to someone that knows what they're doing?"



Contrary to what you might initially think, it’s actually pretty simple and easy to completely restore your black tank sensors. So, we asked our own Master Technician and Blue Moon founder, Rodney Simmons, “Rod Rod the RV Gawd”, how to cure your black tank of doom…


"Yes, there is something you can do!"


  1. Fill the tank completely full of fresh water with a water hose at the bottom of the TOILET with the pump valve closed. The tank will be full when you see water at the bottom of the toilet. We use this method of filling the holding tank to prevent rupturing the tank.

  2. Allow it to set over night. then dump the tank. No chemicals needed!

  3. DO NOT travel with a full Black, Grey, or Freshwater tank! This can damage the tanks or frames that attach them.A couple of gallons (2-5) of fresh water in your tanks are ok and actually help clean the sides of the black tank while driving and a little in the freshwater tank gives you a bit of water for toilet or washing your hands.

  4. After dumping the tanks attach a water hose to the black tank and flush on the outside of the RV. (Make sure the dump valves are open).

  5. Turn on the water via the hose to the black tank flush and run water until only clear water comes out of the sewer hose.

  6. Turn off the water hose supply and remove.

  7. Leave the dump valves open and termination cap off. Allow to dry overnight.


The tank level indicators will now function properly until the tanks are used again. Unfortunately, tank indicators are not reliable, and only accurate when tanks are empty and dry.


- Rodney Simmons - Blue Moon RV's Founder and Master Certified Technician

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