Time2roll" mentioned an expansion tank..not really needed with an RV water heater because of the built in air pocket. Exactly why it is there.
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Kind of a ridiculous statement,...the air pocket IS NOT built in,....its not there,... you have to put it there quite often, and most RV'ers don't do that,
and that's why these excessive pressure problems come up. But, a leak should not happen before the pressure relief opens, so the leak should be fixed.
IMO, RV MFGs should be forced by code to make a small bladder tank part of the water system in all RVs. Until then, if you don't have one in your water system, install one. Normally they are not hard to install, and will serve more than 1 useful purpose.