For the second time on our 2011 24RKS the black water sensor reads that the tank is 1/3 full when it is empty. I am sure that the float is stuck. The first time I took it in, they cleaned the float after taking the tank off. It was expensive. Now it has happened again. We almost always use the campground facilities and rarely use the toilet in the trailer. Is there a product available that will clean or loosen whatever is binding the float? I tried filling the tank with a mixture of bleach and water with no luck. Something stronger has to be available. I hate to have this problem every few trips.