Come on Barry, give Julie break!
Julie, I think you are refering to the Carver Cascade water heater.
After draining the Cascade, replace the drain plug, and close the top valve. (it may already be closed)
Drop your pump into the cold water carrier. Fill and turn on a hot tap. after a few minutes you should get water through the tap, then you know your Cascade is full. It will remain full even if you run out of cold water, so dont worry.
It only empties when you activley use the drain plug and top valve vent on the out side of the caravan