If you are permanently parked, or parked long term without moving the RV, it is difficult to keep the sensors clean of gunk. If you drain your tank completely, then add 5 - 10 gallons of water drive it around, the sloshing on the sensors will clean them. You can throw some laundry soap or dishwashing soap down the toilet too to help clean the tank too. When you stop, drain the tank. If this does not work, you can try the various chemicals as others will mention. I don't have problems with my sensors, because I keep some water in the tanks (3-4 gallons in each tank) when traveling. I've found this is really the best cleaning method. Simple, and costs nothing. Never a problem with mine.