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Hi there, I'm Cole. I think I can help you. So have you shined a light down the toilet too look in there. if its not full than you probably have some stuff stuck on the the sensor in the tank. they do make a solvent for the black water tank that is supposed to remove any gunk on sensor and other buildup. hope this helps i have had this issue in my camper before. if none of those fix it you may have to have the sensors looked at.