All my experience is with older coaches. My last was a 98 Beaver and I sold it at 14 years old, only because I needed the cash for my business. It will run until it is at least 25 unless somebody wrecks it. And I would happily buy another that age without any qualms, provided it was a quality brand like Monaco, Beaver, or Country Coach.
The CAT engine should be checked by a CAT dealer doing a cylinder test where they run the engine on various pairs of cylinders. The same guys should be able to check out the transmission. If all is good, that tests the compression, the fuel injection system, and the electronics. Tires are a big deal and need to be under 5 years old, but if you join FMCA they and some of their brand clubs have good discounts on Michelins.
I wouldn't buy a coach without air levelling. I used to say the same about a hydro-hot or hurricane oil fired hydronic heating system, but these days I think I'd prefer to put in a heat pump system for both heating and A/C.
As far as the aqua-hot goes, they're easy to fix, even if they've been frozen, and oil burners are simple as well, so there is no reason to be scared of them.......on my Beaver the dealer was incompetent and I got the aqua-hot system as a box of parts!
Mine also had some issues with the seals on the slide and sealing between the fibreglass roof and the sidewalls. The sidewall/roof joint was simply a matter of replacing the screws with longer SS ones and using a good sealing caulk in the joint. The seals were a little trickier as I had to remove some trim to do the work, but again it was nothing that a good carpenter couldn't do.
IMHO, you should consider the CC again, although it's probably not worth more than $75K.