IIRC, the water heater is both gas and electric. When you turn the water heater on inside the RV, it will use the electric coils to heat the tank when the demand for hot water is not immediate. When you are using a lot of hot water or boon docking, the water heater will use propane.
Make sure that switch is off when you drain the tank or you can burn up the electric coils. Which you realized.