One of the problems with RV's is the length of the fuller filler pipe and the angle it slants at.. Normally on a car the pipe is fairly short and heavily slanted down to the tank, but on an RV it runs near flat and much farther.
Thus, if the RV is slightly tilted TOWARD the pump.. What you describe happens.
if the RV is slightly tilted AWAY from the pump, better things happen.
I have also had this problem on a van when a vent hose somehow got kinked,
On my class A however,, I will drop the pump side rear jack to tilt the rig a bit if I have to. This helps.