the best improvements to my AC in my trailer were:
1) remove all the ceiling vents and then use metal duct tape, close up any gaps between the vent and ducting then reinstall the ducts
2) drop the main cover around the AC and again, using metal duct tape close up every gap between the AC unit and ducts so ALL the cold air goes down the ducts
3) block off the ducting shortly after the last ceiling vent.
4) following the ducts, I added a total of 4 more ceiling vents in the existing ducting. 2 in the living area, one in the bathroom and one in the bedroom.
5) replaced all the factory ceiling vents with aireoport vents (Amazon). The vents deflect air in a 360 degree circle and can be easily adjusted to increase or decrease airflow through a given duct. a quick twist is all that's needed to open or close them.
Doing the above made a very very noticeable improvement in the AC in the trailer.