What's the difference in height between fill connection and top of tank? I've seen cases where this difference is only a few inches a lots of water slops out. One solution is to install a check valve near the tank if possible. May have to search some for one large enough for the fill line. As for the air vent line you could add a few feet and form a loop that extends afoot or more above the top heigth of the tank. I do not know anything about your model for details.
Fred & Denise
Santa Fe, NM
2007 Excel Classic 30RSO 2007 RAM 3500, Diesel, 6Spd Auto, SWD, 4x4, CC & LB