Sounds like the tank vent may be plugged, giving you back pressure up the fill line while filling. Not being familiar with your rig, can't really help with the where's on vent line routing. My first RV had a similar issue. Fill at more than a slow dribble and the water would just spew back at me. The vent line was not properly secured allowing a low loop which filled with water effectively blocking the vent.
Good luck, hope that helps!
Greg and Karen, the pups Bailey and Lily (St. Bernard mix and Beagle mix) and Lola (the attack cat)