Originally Posted by hubrik
... Can you tell me what's broken here and how best to fix it? ...
While John HD
posted the best starting point, there may be nothing wrong, OR your water heater's overpressure relief valve (OPR) may be bad, OR your cold water supply's OPR valve may be bad OR your regulator may be bad.
Assuming ALL Airstreams are as well built as mine, your shore water plumbing will include, after the shore line, a check valve, a cold water OPR, regulator, and then the water heater's OPR. The water heater's OPR is designed to vent when either the pressure or temperature is too high. The cold water OVP works on pressure alone.
Your water heater, when in operation, will raise the pressure in both the cold water & hot water lines. With a functional check valve, this pressure can not exit through the shore water hose. It has to go somewhere. Ever notice turing a faucet on & the water coming out fast at first then quickly returning to normal? That is pressure caused by heating water.
If there is an air bubble (desired) in the water heater, it will compress & absorb most of the excess pressure. But if your regulator is either misadjusted or not seating well, your lines may have too much pressure if the shore water's supply pressure is high. Or, as John indicated, you might be out of air.
It is possible that your cold water OPR used to be venting this pressure, and has now stopped for some reason.
Start with John's remedy first.