Probably Vibration ?
My first guess would be engine vibration disengaging a worn breaker. See if it pops out when you tap on it with the butt-end of a screwdriver or sometnhing. If it does, then its bad. If I read you right, this is the thermal switch that breaks the AC circuit in the case of an overheat. With the engine on and shore power disconnected, there is simply nothing on this circuit. Does it only happen with shore power connected ? With the water heater main breaker engaged ? disengaged ? Answering those questions will help isolate whether its a physical or electrical issue. Hope this helps. Duane
