I've been RVing since 2007, and black tank sensors are useless and can be a waste of calgon, dishwasher soap, ice cubes, and other various tank cleaners; the way to gauge the black tank is to look down into it through the toilet when it is empty; as the toilet is used, count flushes to keep track of how many toilet fills of waste go into the black tank; to make sure that it drains as much as possible when you dump, use A LOT OF WATER!
As we were taught in elementary science, water is the universal solvent; to allow our holding tank to have the greatest capacity after we "dump" is to use lot of water and a deodorant; this is the best way to maintain our black tank. Water, and a deodorant is the only way I do it.
By the way, my black tank is currently empty, the guage says that it's full; it's useless...
