Very common problem, especially with jayco and Starcraft models. Fortunately also very easy to fix. Overflow tubes underneath begin leaking long before tank is full and continue to do so until up to 1/3 tank is drained. Often happens in transit...you fill up at home, arrive for a boondocks, only to find you lost a bunch of water. Valves need to be put on those overflow tubes. Have the valves open when filling, but as soon as your tank registers 100 percent full, close the valves. Solves the problem. Good luck. Lots of us found this goofy situation and have fixed it quite simply.
Those tubes are vents. If your tank is filled by using an attached garden hose, you will do damage if the added valves are closed. Likewise, using water with the vents closed will collapse the tank. Know how your system works before following the advice of others.