The water heater will be on the same circuit via the RCD as possibly two other electrical appliances so make sure it's not something else on the same circuit that's tripping out. If it is the water heater then it sounds like the heater element has malfunctioned and needs replacing.
Maybe it does work on gas but the it's not going to trip the electrical circuit being operated by gas is it?
Your thread is related to an electrical problem which has nothing to do with the gas supply to the heater.
Of course it will work on gas as it is not on mains voltage. It is working off the 12 volt leisure battery and the gas only. Have you actually checked your mains hook up cable that is has not got an earth fault on it.
Perhaps if you could be more specific about what is actually tripping it would be easier to help.
Is it:
MCB in on site bollard
Circuit breaker in the house.
MCB in the caravan consumer unit
Main switch in teh caravan consumer unit.
Water heater itself.
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