Some chatter about 60s trailers and roof air came back around recently. I struggled with the decision for awhile the year we got the Overlander.
Since it was easily available on CL...I bought a frigidaire window unit. Using a small wooden snack table, I mounted the unit temporarily in the front window.
Removed the screen, and made a surround out of heavy clear vinyl. The vinyl attaches to Velcro strips inside the trailer around the window. The a/c unit sits on the table which has its front legs resting on the LP gas tanks. Rear legs are dust off leaving a small wooden plank that goes over the window ledge. It has a cut out in the middle so the plank rests flat when going over the window frame.
Works quite well in keeping the trailer cool, except during the day when it is well up in I the 90s. Then with the addition of a vintage fan in the kitchen, it's comfortable inside for our little dog to snooze while we see the sites. At night with temps outside in the low 80s or below, we sleep,quite well with inside temps getting to the low 70s.
No extra holes in the roof, and the whole ordeal cost us 50 bucks.
Unit travels in the SUV when we roll, and stays home much of the year.
A Fantastic Fan in the bathroom helps year round. Kudos to Airforum member RideAir for the idea!