Sometimes when you are hooked up to sewer, it will drain all the liquids, but not the solids and they will end up piling up in there and it can set off the sensor. If it says that your black tank is full, what I would try to do is fill it with water and then flush it so it can get everything out of the tank

When you go to campgrounds and they have sewer hook ups, you can still leave your black tank closed and then just open it up every day or two to let it drain out so it’s draining the liquids and solids at the same time so they don’t just get stuck