On my old Teton (2010) all installation is done from the roof. It is held in place by big a** screws into the roof framing. The challenge for me was that I did not know that the replacement A/C had different ducting. I had to modify the cold air outlet of the A/C to fit the ducting of the Teton to prevent cold air mixing with the return air. I also had a problem with the wiring until I discovered an adapter box that matched the new A/C to the existing (old) wiring on the trailer. Once I installed that and correctly set the DIP switches, I was good to go.
Hope that helps.
Catfish