Fill your water heater the same when on City or Pump. Make sure the by-pass valve is off so the tank will fill. Start the pump, or turn on the water from the outside spigot. Go inside the camper and turn on the hot water faucet at the sink, or the shower. Let it spit and spew and squeek and squalk until there is a steady stream of water. The tank is full once you have a steady stream and the "spitting" stops.
THEN... turn on the water heater .... either gas or electric (After it's filled). If your tank is not filled and you turn on the electric, it will burn up the electric element. If you flip on the gas, it will heat the tank, but could damage it. So when turning on the heat, always make sure the tank is full. When draining, always make sure the power or gas is turned off before draining.