Are you sure you're using the water fill and not the fresh water hookup? On my 2005 BT Cruiser 5250 they are in the same compartment but I understand that is not the case with all models. In photo I attached, water fill is at left, water hookup at right, and vent for fill is in the center below them. And make sure the water fill hose inside the rig hasn't slipped too low to fill the tank. Also, though it's never happened to mine, from the Facebook Gulfstream group I remember reading that if the vent gets plugged it will be difficult or impossible to add water.