As stated, you have "HOLDING" tanks, they are designed to hold, "stuff", until full. Then you dump them. Always use plenty of water, black tanks start with about 7 gallons, then make sure you use plenty of water when you flush. As far as the faulty indicator, it can be frustrating, after using the RV for a while, you get a pretty good idea of when you need to dump the tanks. You may try the "George" method, your starting with the 7 gallons in the black tank, a 5 gallon bucket fill with hot water, add 1 cup calgon water softener, and 5 drops of Dawn dishwashing soap. Flush that down the toilet, then drive to your next destination. The sloshing while driving should help clean the sensors in the black tank. First thing when you arrive, dump the back tank, use the sprayer, dump again. Your sensor should now read empty. If it doesn't you may have a bad sensor. Good luck!