I installed a Fridgidaire unit from Lowes to my new 2014 Scamp
16, it fits to the closet very well.
I did not cut through fiberglass on the trailer side, but made both intake and outtake holes in the floor, separating the air streams as much as possible.
The 5000 BTU unit doesn't have drain hole, water is supposed to be removed by the fan with hot outtake air.
Small 12v fan from wallmart makes a good job blowing the cold air from the low lying unit through the trailer interior. Total cost (A/C plus extra fan plus some lumber and vents for installation plus AC wires and outlet) was just below $200. The labor (mine) costs nothing