I really think the opposite is true, of what a previous poster stated regarding opening and closing a valve. I believe that in most motorhomes, once you are connected to city water and have the water turned on, it is automatic, without the need for turning any valves. Make sure there is no obstruction of a filter on the inlet side of the motorhome. What is curious, is why, if you had the water pump off, the tank would empty. There is no way that the water would flow without the pump being on, at least in my opinion. Did water flow past the regulator? Try it without the regulator.