Black tank

Wasps are notorious for building their nests in the vent tube. If your toilet 'burps' your vent is clogged. Could be a birds nest also. You'll have to take your hose up on the roof with you and 'backflush' the vent tube. Use caution on the roof especially if it gets wet. You don't want to fall. The vent tube feeds into the black tank so either watch how much water you send down the pipe or leave your black tank handle pulled. If you leave it closed and fill the black tank it will flow out the toilet into the bath. Another mess you'll have to clean up.....Dennis