I, too, lost about a quarter of a tank of fresh water through the overflow while traveling. Thanks to the input on this thread, I put a ball valve in my overflow line and traveled this past week with the valve closed --- didn't lose a drop of water! Just remember to open the valve when filling the fresh water tank or the trapped air won't have any place to go and you could split your tank. Also, remember to open the valve upon arrival at your destination so that air can get back into the tank and you aren't trying to pump water with a vacuum.
I can't believe Forest River installed tanks with this type of overflow setup!
2002 Windsong - 34'