My previous trailer had a similar problem. Turned out tube inside the wall of the trailer (from the fill hole to the tank) was routed up, then down (sort of like an upside down trap under a sink). Changing the routing so it went down solved the problem - it filled very easily after that. Not sure if I described this very well - hope it helps...
2012 Rockwood 8281SS (Mods: LED's, Storage Thumb Locks)
2009 Chevy Silverado LT 2500HD Diesel w/ Allison Tranny