Since I often camp where there is no water available I have to fill my tank before I leave as well. I had the same problem then I found that for some reason Jayco put an overflow right on the tank and it acts like a siphon. When it starts to work it will pull out over half the tank. so I plugged that overflow with a plastic bolt and just rely on the overflow at the fill spout now.
2004 Chev Silverado Duramax optioned past the max. 2009 Jayco Eagle 308 RLS 765 watts of solar, 6-6 volt batteries (696 amp hour), 2000 watt (4000 surge) whole house inverter.
140 days boondocking in 2016
211/2015, 196/14, 247/13, 193/12