I have a dump at home and before I park it for the summer, (Phoenix) I dump & flush the tanks. On the last monthly maintenance run, the black tank showed 1/3 full. Unless I have a bunch of meeses using the pot, I'm inclined to ignore the sensor. I usually dump on an "It's about time" schedule. When we do use the toy hauler it's off road and gets really "all shook up". I will admit, before I wised up to the reliability of the tank sensors, when camping with the family and the black tank light showed full it got a bit worrisome. I thought I had miscalculated. I did all the water softener/soap theories but I didn't see it as a remedy. The best sensor is to shut off the pump, open the flush valve in the pot and take a peek.. IMO ....