I used a method I had spotted on AirFourms for installing a temporary window unit.The member was kind enough to email his directions...I then found a Frigidaire 6500BTU in the middle of Winter, a good time to buy one.
Using a small wooden outdoor table, you trim the rear legs off, these actually unbolted. Then the front legs rest on your LP tanks, while the back has a small lip that goes over the window frame. It fit perfectly. I added two locking loops to extra secure the logs on the tanks.
The unit is 40 lbs. Sits on the table, with the drain well out the window. I then use clear 6 mil plastic film to surround the unit , held on with velcro. No permanent mount, no roof unit and the trailer retains its look..
With it 90 degrees out yesterday ( in early MAY..yikes). It got down to a bit over 70 inside. Bonus is that the unit is quiet and has a remote.
The Forums member disguised his with a black open weave fabric over it, but I am not totally unhappy with the look and may leave it alone.
The best news is that for under 100 bucks I have cold air inside and still retain the look of the classic shape.