Fresh water tank.

I had a very similar experience. Began to occur after I had filled the tank and then towed the camper. I eventually cupped my hand around the water inlet and basically pressurized the tank until the vent line spit out water and then there was a sort of burbing and air began rushing into the vent line and all was right with the world again. I can't see the entire vent line, so the only thing I can think of is water was sloshed into the vent line while traveling and sat there until I forced it out. You could probably use air to make sure the vent is open as opposed to forcing water into the tank. But be careful not to use greater than 35 psi regardless of which approach you try or you'll have a much bigger problem on your hands.