Should be a quick fix. I had this problem on another trailer. Turns out the hose (inside connected to that outside fill) was too long, and the water would fill the hose and only (slowly) go into the tank. (ie...the water took the easiest path, which was right back out where it came from.
Track down this hose inside your trailer to see if it's positioned correctly. If not, reposition it so it works, or simply cut it off and reclamp it. (shorten hose) Once I did that, I never had a problem with it again. Hopefully, that's the only thing wrong with yours too.
Let us know if you find the same problem.
PS...there is usually a tank vent hose connected to that same outlet fill. You can identify it by the small screen over the hole outside right near the fill. Make sure the hose is connected, or it's not plugged, blocked, or kinked. This could prevent air from escaping as well slowing the process.
Pete (Jolly Mon)
2018 Wildwood Grand Lodge 42 DLTS
-Seasonal Space, Northern MN.
-2017 Sylvan 8522 LZ LES/115 Merc/Sea Legs