Fresh water tank fill issue

We bought a 2013 Wildwood with the same problem. The reason it was built this way was because the fresh water tank is under the walk around bed and the water has to be under pressure to push it under the floor and back up to the fresh water tank. If you look at where that outside water inlet is and then follow it to see where it goes inside the trailer I think you might find the same thing. I went to Home Depot and bought a 4" sewer drain clean out w/cap, like you would find on the outside of your home, and cut a hole in the top of the water tank and glued it into place with caulking useing my caulking gun. It turned out to be pretty easy to pour water into that 4" opening. the only downside to this is that you have to take the water inside to pour it into the tank. I hope you find this helpful.