Here is a link to another thread that deals with this extensively.
You can skip the first 4 or 5 pages, as they contain mostly people refuting the OP's assertion that the tanks lose water and siphon. Those of us that have/had this problem know it is true! The last several pages discuss solutions.
A couple things to remember to do after you have installed valves or clamps on the overflow tubes:
1. ALWAYS have the valves open while you are filling. On several Jayco models, especially those with 90+ gallon capacity, there are TWO fresh water tanks. The gravity fill goes into the first tank, and a set of hoses between the tanks equalizes the water level. But only the first tank has a dedicated vent to allow air to expel as the tank fills with water. The second tank uses the overflow tube as an air vent. Once your tank is filled and water begins to flow from the overflow, that is the time to stop filling and close the valve, while you travel.
2. Similarly, when you arrive at your site, it is important to RE-OPEN the valve on your overflow. Again, it is that second FW tank that needs to get air as your water pump draws water from your tanks. Because the tanks are full, the hoses between the two tanks are filled with water and air cannot get to the second tank other than via the overflow. It probably will not actually hurt anything, but I forgot to open them up recently, and when we were finally leaving our site and I did open the valve, there was a couple of minutes of gurgling, as the 2nd tank sucked in air, so I definitely had negative pressure in that 2nd tank.