I would first swing the door towards the closed position and try and note where the resistance changes from hinge friction to resistance because of the gasket compressing. The large the angle at which you note resistance from the gasket will give you an idea of where the problem is.
If that angle is say greater than 10 degrees the hinge side of the gasket is being compressed TOO soon and could be reduced if you pad out the hinges against the trailer body.
If the door almost reaches the latch and a last heavy nudge will latch it you may be able to just adjust the strike plate, on the door frame, out to reduce the effort necessary to latch the door.
As a last resort you could file the tongue back. If you do this be sure not to introduce a taper to the face of the tongue as this will keep the latch from seating all the way and may result in the door flying open while on the road.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles