hoegee -
the line at the top of the tank is for overflow (as i described in my previous post) - its there for a reason , but it should only allow for a inch or so of run off or sloshing. if you're loosing a lot then the overflow could be creating a siphon effect (hopefully they don't have a tube into the tank that would enable this).
also make certain your fresh water tank/drain valve is completely shut & is not leaking.
I always fill my tanks at my residence - because i trust/know my water supply & what the faucets & hoses have been used for. If you can find a trusted water supply closer to where you dry camp then that could allow you to fill the tanks completely full w/ less loss and allow you reduce your trip/carry weight & possibly even increase your trip/mpg.