Something to try, remove the closest supply discharge defuser to the unit, use a flash light and lay it in the duct then take an inspection mirror and have a look inside, see if there is anything there.
Another thing you can do at the same location is to take a wet vac and put it on reverse to where the air is blowing out and stick it in the vent, but remove the grill on the a/c unit from the ceiling first.
If there is a blockage you may have to force air into the duct work in the opposit direction.
Now before you do all of this i suggest to get up on the roof and remove the cover to the unit and take a look, there is a small piece of duct work that is attached to the supply side of the unit to the duct work of the camper, if it came apart then most likely the supply air is being discharge around the unit and blowing out side. One quick way to tell, if you get on the roof while the a/c is on and you fell cold air being discharged around the unit then that will tell you where your a/c is going.
The "Fast Cool" never works right for me because of two reasons 1. the supply and return is right next each other and 2. the location of the T-stat, a large amount air being dumped in one spot will cool the area around the T-stat but will not cool the entire space of the camper. The 13,500 BTU unit was not enough to cool my 34' Rock Wood above90 degrees, i installed a 15,000 BTU Carries unit and does a great job.