Flybob, I put a shutoff valve on the vent/overflow line to save water. We stay at a state park for 4 - 5 days once a month plus our other trips. They have electric only on their sites. When I would fill up the freshwater tank, the water would slosh around in the tank and come out the vent/overflow line. By the time I would get to my site I would have lost 4 - 5 gallons of water. Now when I fill up the tank, and the water comes out the vent, I know the tank is full. I shut the valve on the vent line and proceed to my site and do the set up chores. By then the water is not sloshing in the tank and I open the shutoff on the vent line. Only water that comes out now is the water trapped in the pvc line. Yes, I realize what can happen if the vent line is closed while filling - you make a 40 gallon water tank into a 50 gallon tank.