I should have taken a picture when it was mounted (next time). I bought the AC-80 support bracket from the home improvement store:
Amazon.com: Air Conditioning Window Unit Light Duty Support Bracket - up to 80 Pounds: Home & Kitchen
and a small bag of the black foam pipe insulation. A couple of SS bolts with locking nuts to have the "lip" of the bracket held on (the default install assumes you are screwing the lip into the window sill). I lined the window opening with the pipe insulation split lengthwise, slap the AC unit onto the bracket (which is nicely adjustable with a level for the proper tilt back for drainage). Once you have the bracket setup for the proper tilt, that never varies again so you don't have to ever mess with it again - it just folds down nicely for storage when you remove it. I cut a few pieces of 1x4" and that fits in the top "rail" of the unit on the inside and mates with some 6" threaded bolts with wing nuts onto the same size piece on the outside of the camper. Then I just wedge a few pieces of pipe insulation in anything else I can see through.
The 5000 btu units are overkill for the 13' scamps so the thing doesn't need to be air tight. I guess if I wanted it to survive the occasional rain storm without a leak I'd be more picky following a few of the really nice fabricated setups others have posted.
The Frigidaire FRA052XT7 works great and is cheap enough that's for sure. And I found it much more quiet than I expected it to be.
I assumed I needed to mess around with bumper pole support sticks and stuff, but honestly the unit is just not that heavy and the bracket transfers a lot of the weight
to the area down near the belly band which seems plenty strong enough. I don't need to drive around with it mounted so not worried about that sort of stress.