Mine did the same thing and it several trips to figure out the water was coming out of the over flow hose especially when going up hill or taking corners. I just routed the overflow hose about 4-6 ins above the fitting on the tank which is on the side rather than on the top over the tank. This fixed the problem. I would not install a shut off fitting in the line. I saw a guy forget to open the valve when filling his tank and the expansion of the tank broke the attachments to the frame. It took myself and another camper all afternoon to make a FG repair and new steel angles to fix it.