The water fill cap on my Packa Potti has vent slits on the outside and a 1/16th hole on the inside. Can see a rubber through the hole. SSSSSSSSSOOOOOOOOO the fresh water side of the (MY) potti is vented by design. My fresh water leaking was happened when I "overfilled" the water tank. Got the water level above the bellows to housing seal. Then I got some drips to water on my rear RUBBERIZED floor
Look close at your fill cap.
The "gunk" on the replacement bellows was really sticky. Was covered with a waxed paper that was removed just prior to installing the bellows. So buy sticky gunk. Sorry, that is the only advice I can give. Not sure what store sells "sticky gunk".