The air gap is actually an expansion tank. Water being uncompressible will rapidly increase in pressure when heated. If there is no air bubble at the top of the water heater, then the water pressure will pop the P/T valve every time the burner or element kicks on. With the air bubble in the top of the tank, it just compresses the bubble. Re-establish the bubble by shutting off water to the camper, open the P/T valve and drain some hot water. Close the P/T valve and open things back up.
Skip - if the bubble keeps collapsing, I'd replace the P/T valve. The seat isn't holding.
Rich - you've probably never had to deal with this because you drain your water heater every fall, thereby re-establishing an air bubble every spring.
