My freshwater tank also drained on my first trip. The fill hose and the overflow/vent are at the same level on the side of the tank. Initially I put a valve on the overflow/vent as it just ran straight toward the ground. I would open it to fill the tank and then close it for traveling. You need to open it again after you set camp to allow air into the tank to replace the water you've drawn out. Actually, the overflow/vent should be open above the tank and then there is no siphoning. The shut-off valve (think garden hose end) must be open when filling and using the freshwater from the tank to avoid over-pressure and vacuum conditions that distort the tank. I've since routed the vent hose up through a floor opening behind a galley cabinet into the area of the outside shower which I removed (I never use that anyway). This solution also allows the tank to be filled rather than overflow an inch or two short of full. The problem seems to be that the trailers with slides have limited ability to vent above the tank if the tank is mounted below the slide out.
