I just dewinterized my 5er a few weeks ago. I filled the fresh water tank and added bleach to sanitize it. When the tank was filled I turned the water pump on and it ran for the longest time even though before using it I had connected to city water first and the water lines should have been pressurized. So I looked at the pump and the line leading into it and realized that I hadn't turned the valve from adding antifreeze to the fresh water tank. As soon as I turned the valve to the fresh water tank the pump pressurized the system and shut off after a short time. Could this be your problem?