You will find very few if indeed any diesel class C's in the range you're looking at. The diesels available are generally either based on the Sprinter platform (which would be a bit undersized for most in your length range) or else super-C's based on heavier truck chassis. The vast majority you'll be looking at will be on Ford or Chevy van chassis, more often Fords, and these are gas only (unless quite old).
That being said, you're very unlikely to ever put enough mileage on a motorhome to wear out the engine. Both the Ford V10 and the Chevy V8 are quite reliable and well-proven engines that will typically run a long time without significant problems if given reasonable basic maintenance.
The main tradeoff for a shorter model is the obvious one of interior space and hence appointments. Much below 27' and you won't find a center bed in the back (mainly because there isn't room enough in back of the wheel wells unless the rear overhang is excessive). With any class C, it is very wise to check the weight and balance and cargo carrying limitations; some have not much payload at all left for you and your stuff.
Do of course take the unit for a test drive. The motorhome will definitely not be as maneuverable as your truck, and most likely slower overall. Class C's tend to have pretty abysmal minimum turning radii, unsurprisingly worse as they get longer.
A fair number of people here have gotten and been very satisfied with ex-rental class C's from e.g. CruiseAmerica. There's a long thread on these units here. It's an option that IMHO is worth considering; the prices are generally fairly competitive, and the rental companies (at least the major ones) do a fairly decent job of maintenance.
While it is likely a little bit simpler to hop in a class C and go camping than it is to hook up a fifth wheel and head out, there's still a good bit of prep work involved. It generally takes me at least a few hours to fill the water tank, check the tire pressures, put food in the fridge and cupboards, get clothes together and stashed, and so on. I would give some hard thought to how much you'd actually gain in additional use and see if it's worth the financial hit you're (apparently) going to be taking.