Restriction of the ducts will result in the symptoms you describe. If the a/c is only putting out half its rated airflow of 55 degree air it will never cool the trailer. Also, I've seen duct work come loose and dump all that cold air into the space between the ceiling and the roof. That won't go far toward cooling the trailer.
If you have a snake camera I'd stick it down thru all the vents and ductwork looking for leaks or obstructions. I sure wouldn't intentionally obstruct any of the outlets.
I have a 21' trailer (with two large slides) with a 15K BTU a/c and I can get it so cold you need blankets watching TV when it 100 outside. Granted your trailer is longer but it seems to me it should cool better than that.
Also, some units come from the factory with a poor sealing job between the return and supply chambers in the a/c itself. That seal (usually just tape) can deteriorate over time and cause cooled air to simply recirculate within the a/c and provide no cooling to the TT. Pull your filter panel to take a look at that area.
