Hi Gary. Steven makes a good point. Our old Pace Arrow had a similar issue. The vent and filler hoses were quite long and over time had slumped enough that the filler's fittings were stressed enough to leak, and the vent had enough belly that fuel accumulated and blocked it a bit. As I recall I added support strapping to counter the sagging and spliced in a short section of filler hose to facilitate less stress on the leak-prone end.
I'd check the configuration and condition of any vent hose, its entire length. If it's formed a belly, been pinched off somewhere, or is otherwise even partially plugged, it would explain the filling issue. You might initially see if applying compressed air from its end blows any blockage back into the tank.
Joel