If one can install themself, you can buy a minisplit for under $800. You can't buy a rooftop for under $800 that I am aware of. My Coleman Mach 15K is $1282 on Amazon right now. That is 50% more expensive. Installation isn't that complicated, especially if you buy a precharged unit that doesn't require pulling a vacuum. The wires, condensate line and refrigerant line all run together. I've seen them run in pantry cabinets, behind curtains or tucked in the corner of a wall with a small cap over them. Someone with moderate skill can install one in a couple hours with handheld tools.
Even paying for installation, an HVAC tech will charge a couple hundred dollars for a minisplit. A RV shop will likely charge at least that, if not more. Granted, it is a retrofit so you'll have to do some work to find a place to put everything, but that doesn't make it more expensive (which is the point you are refuting).
And I wholeheartedly disagree that a minisplit isn't massively more efficient than a rooftop unit! They draw significantly less power than a rooftop AND cool more quickly than a rooftop. They also maintain temperature much better than a rooftop. Compare the yellow energy usage sticker from a rooftop unit to a minisplit, significant difference.