Are you familiar with the process for by-passing the water heater? By-passing the water heater is done in cold climates so that antifreeze can be pumped through the rv water system without filling the water heater with antifreeze (this is necessary so the lines don’t freeze and break). The point being, the by-pass valves need to be in a specific position to accomplish this by-pass mode. If those valves are currently in by-pass mode, you won’t get any hot water. If you don’t know about “by-passing”, google it and read up.