At 1 o'clock to the hose hole you should see a 3/8 dia hole that may still have the plastic plug in it. That is the vent. Remove the plug while filling the tank.
If that is not the problem the vent hose may be kinked. Try blowing air into the vent hole with a compressor and see if you hear air coming back out the fill port.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles