Check and see if you have a valve that shifts water supply from the hose to the tank. Don't run the pump too long without water, it can ruin the pump. I'd start by filling the on board tank. Look for the valve I mentioned, (probably labeled differently) Also, do you have a hose and valve attached to the pump so you can winterize by dipping hose in anti-freeze bottle? If that valve is open it will only suck air.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI