The fact that this RV has a "screw in" fill leads me to think that pressure is your problem. You are displacing air with water, that air has to go somewhere. Without the ability to vent air it's building pressure which is slowing your flow down. Aside from your filling issues I wonder how much risk this is inviting with pressure building. That, and I would think there should be a vent somewhere to keep this from happening. I know on most gravity fills there is a small vent hole in the fill housing itself. Maybe try not screwing it in so tight to let some air escape.
