Have you checked your water heater bypass? RVs have a system of valves that divert the water around the water heater for winter storage. Before you turn on the water heater you should make sure the valve(s) are set so the water runs through the heater.
If you run the heater with the valve(s) set to bypass you will damage the heater. Actually, running the heater with the heater tank empty will damage the heater.
There are several configurations of the bypass valve(s). I can't tell you what yours is like.
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