I've read up on all the others mini-split installs out there, so I thought I should add to those great write ups with my recent experience.
Went with a 120v Pioneer 12k inverter++ unit (works down to -13, and it is mostly to keep the bedroom toasty in the Maine winter). All the installation materials can be ordered from Amazon or Home Depot, ended up costing $1,300 all in. Had to do some very basic wiring (make sure to check and double check) and use a cheap vacuum pump to clear the lines. They recommend a torque wrench when attaching the copper lines, but I just snugged them up and made sure the vacuum held, easy peasy. I had no experience doing this sort of thing, and it wasn't difficult. Completed in two consecutive afternoons. Plenty of good youtube videos on people installing these DIY mini-split units (though not in an Airstream!).
Attached a photo of the outdoor unit, the lines are long enough that it can be mounted on the bumper when Stella hits the road again, but she'll be in place all winter so I bought a cheap pad/stand for the time being.
Ran all the lines through the access panel (had to put a 3 inch hole) and the outdoor unit power was run through the hole that was already in place for the locking mechanism.
Mounted the indoor unit low, below the window. There was one rib that I could drill into below the window, but just to be safe I also put in two aluminum bars between three ribs up top and two ribs below to rivet the mounting template onto. The indoor unit isn't very heavy, and feels really solid.
The mini split outdoor unit requires a dedicated 20 amp breaker, so I wired it to the breaker that had been used for the Dometic AC unit (which I disconnected for now). In the future the mini split could have a dedicated line that could be plugged into its own 120v shore power outlet, so it could be plugged into an outlet at an RV park. Didn't want to run another long extension cord, which is why I wired it into the Airstream for now.
The unit has been running flawlessly for a few days as a test. Spent the extra $40 on the wifi module, so I can control the settings etc from my phone. Nice to be able to turn the heat up or down remotely. The real proof will be how it holds up over the winter. Will hopefully get the new Suburban propane furnace up and running soon so there will be a backup in case of a power outage etc.
Planning on installing a 12v
caframo sirocco ii fan tof push air from the back of the airstream towards the front. There is a cigarette lighter plug under the indoor unit (with a purple wire) that I'm hoping I can use for the fan, otherwise I'll use one of the overhead reading lights.
Happy to report back as things go along. Also happy to answer any questions to anyone thinking of doing something similar. We have Mitsubishi mini splits in our home that are our sole heating cooling in an old 1790 farmhouse that work flawlessly, so I'm hoping this works just as well for the Airstream!