There are probably multiple contributing factors. The tilt of the coach itself is one - often imperceptible buy even a tiny lean to one side causes extra problems. If necessary, you might have to deploy jacks on one side (or both). The tilt doesn't necessarily have to be toward the vent either.
Air and diesel bubbles (foam) are a major contributor, so pumping at the slowest possible speed usually helps. Unlike gas, diesel foams easily when rushing in. Fuel runs out of the filler tube into the tank, foams up and rushes across to the vent and exits. It's a fairly common problem.
There isn't a whole lot you can do, though. Because of Class A layout constraints, the filler tube and vent go into the tank at a very shallow angle, so splash back in either is likely. You may just have to post the wife outside to watch for overflows.