I considered many different options but they all had pros and cons. I ended up taking just the cons and ranked them. After thinking long and hard I didn't want the hassle of bringing an AC unit with baffle, hoses, stand etc with me just to set up, break down and store etc so decided to just build it in.
Wow, it was very hard getting myself to cut that hole, I stood around pondering for a good 2 hours before I actually did it! That was after weeks on pondering it already

Pretty straight forward, the only thing I can say that might help others is that every unit has slight variations that you won't really know until you have it in hand so each install will be a little different. It would have been great if it was a perfectly flush smooth rectangle but they aren't.
I cut the hole as small as possible, the opening is flush with a shelf I have inside. This provided extra stability and I locked it in using lips to mimic a sill and trim to lock over the mounting bar. I taped around the hole, then added closed cell weather seal around the outside and installed aluminum angle with compression around that with butyl etc. The top angle extends past each side to form a gutter of sorts to help divert water while the sides extend about an inch past the bottom to make sure water from the sides flow past the opening. Next is the use poly board to fashion a box that will go over the unit and seal against the closed cell foam under the angle for storage and travel. This should create a nice seal since it will serve as a gasket for the box cover.
Tested in 110f full sun weather, and I'm happy to report that it will turn it in to an ice box if I want. Very happy with performance. Our camper is insulated in the roof and floor. I already know it stays warm well.
Very pleased that I found a black unit, goes well with the overall build. Picked it up on double secret clearance at Wally world for $100 flat.