Mini Split AC

I wouldn't mount it flat. That would restrict the air flow/cooling too much.

Most residential mini-splits also do. Good job of heating thus eliminating the need of a furnace and one less appliance that needs propane. If you seldom boondock, and/or are willing to heat water with electricity, you have eliminated another need for propane. Cooking with electricity is a "no brainer".

The point of that discussion is, with no propane, you now have a good mounting location for the mini-split compressor !

Possible not likely due to debris kicking up from roadway. Most have been mounted on tongue or back bumper.

I've seen a few mounted on the rear bumper. Do make sure to beef up the supports as the rear bumper is really just sheet metal not designed to support much weight.

The big advantage I see is it eliminates a hole in the roof.

I would consider the rear of the Rig (not necessarly on the bumper depending on so many things I do not wish to try and list) but make sure the unit has VERY good support.
Same with the indoor half of the split.

I don’t believe the compressor vibration mounts would survive mounting the outdoor unit horizontal, at least not the Mitsubishi units I install.

I modded a 8k window unit for my little Toyota to run with a Honda 2000. I have 3 DIY minis in my house and would have installed a mini in my RV years ago. But I have no room for the evaporator, inside unit. The blower wheel may have to be removed from the AC and on mine need to be pulled out of the left hand side, varies by manufacturer. Removing entire unit is an option. Which means the space on the left needs to be as wide as the unit. Other than that the inverter minis would be ideal!

For my Toy the only area to mount would be someplace on the rear.https://www.google.com/search?client=firefox-b-1-d&q=How+to+remove+blower+wheel+from+Daikin+Mini+Split%3F&stick=H4sIAAAAAAAAAOOwUWJzSczMzswz0srIL1coyVcoSs3NL0tVSMrJL08tUijPSE3NUUgrys9VyM3My1QoLsjJLFnEauRBQDHEUAVfkJ5gkB77JjY7Lna30qzMkuJSIWNC-qEKkSwFGuDMxQOWdE4sKspMLSJsClQhqjNcuLh8QWYnZRZnZAqZETIjF64W1TE2XNCQEyIrMCy4WEOKEvNShfTx6M7PUygBKUK12YiLPSAzPy8VGALqUAaSAlQTwKYm5jSxGXOxuBelpgppo1qXDhTDpdl-AhsjAL-ua5khAgAA&sa=X#kpvalbx=_NL1NYZXrBbeGwbkPwYi9yAg5