I have a 10 gallon Surburban heater that does not heat the water as hot as it does on gas. Showering on electric only only requires the hot water valve to be opened. When on gas it operates with both ...
If the element is heating the water, it's working. It's more likely the thermostat is faulty. If it was a residential unit I would say it just needs an adjustment but I don't *think* RV WH's have them.