Under the outside sink there is a one way Hepvo drain trap intended to eliminate the need for a conventional p-trap. I had to travel with the grey tank pretty full and found a lot of water in the outside sink at the next stop. I disassembled the Hepvo (which Thor had installed upside down) an discovered that the diaphragm had turned inside out and was allowing backflow. Since then I keep the outside sink drain closed when not in use to prevent possible backflow.
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