I am in total agreement with Jack regarding "RV" A/C
--- Having owned a couple of them is what put me on the path to using a window unit in this build. As noted, you can buy a half dozen or more high efficiency window units for the price of one "RV roof mount". But price is just part of the story. There is that big, leaky hole in the roof...the need for grossly overpriced service (when you can find it)...during which there is that big open hole in the roof to contend with...the increased chance of a tree limb or such making said hole even bigger...and on and on.
I went with a little 8Kbtu, $160.00, Energy Star unit that I can replace in 10 minutes at any Wally World in America. And they can be fitted just about anywhere. Mine is going partially in the roof, but is well protected by the element below which just about any sheet metal shop can fab for a few bucks. A roof deck will eventually fit over the flat part. Admittedly, I invested a little extra time & $ to keep the Art Deco look going, but they can be easily installed into a rear window or above a rear door. There is a stainless steel vent hood filter just inside the opening to keep bugs and critter outdoors.
And for you fortunate folks with a basement in your bus...a much larger or even multiple units can be installed below deck and the air piped where you need it. No biggie.