Our Journey has a spring actuated check valve at the fill point. We can check to ensure it is not "stuck" or fouled with grit by inserting a short length of 3/8" wood dowel and pressing on it. I believe it takes 40psi to open it. If the dowel test does not indicate valve fouling, try filling without a pressure reducer. You are new enough, you should be using 50 - 55lb reducer.
No offense. Make sure your "fill/normal" valve is oriented fully in the "fill" direction. If it is charging your internal piping, it is not.
Like you, we have only one fill point, no gravity fill. There is a vent on top of the potable water tank, but to reach it you need to be double jointed, with bones of rubber and much thinner than this knuckle dragging old flange head is these days.
Fair Winds and Following Seas