You might want to ask a moderator to move this to the class C forum if that's what you're looking at primarily. Many class A motorhomes are around or above the top of your length limits.
My general suggestions: I would strongly recommend buying something used to begin with. The initial depreciation on RVs is significant, and as (apparently) first time buyers, you're at a bit of a disadvantage for knowing exactly what you want to look for and what works best for you. There are many things and preferences that you really only find out with some use. If you find after a year or so that what you have doesn't really meet your needs and wants as well as you had initially supposed, you won't take so bad of a financial hit when selling it.
Along those same lines, I would suggest renting an RV for awhile before buying one, even if it's not quite the same sort of thing as you'd buy.
Look for a usable layout first of all. Since you'll be traveling on the road a fair amount (it sounds), rather than staying in one spot for a good length of time, pay some attention to how usable things are with any slides pulled in while in the travel mode. Access to some areas is sometimes quite restricted, occasionally making it very hard to get to the restroom or fridge without extending a slide.
For used units, look at condition, and particularly the care spent preventing leaks. Water damage tends to be quite costly and/or time consuming to repair, and is often more extensive than it first appears. Some roof designs are better than others at keeping water out, but all can work and all need at least some periodic attention.
The differences between specific brands are not as huge as might be imagined. Certainly some are built to higher standards than others, and there can be differences in construction techniques and workmanship with different brands (and different lines within a brand), but many of the basics are much the same regardless. The appliances and windows and whatnot come from the same few companies, there are only a few different chassis and drivetrains, etc.
Gas or diesel engines, in and of themselves, are way down on the list of things I worry about. As it turns out, though, the engines that are available correspond very closely to some of the broad divisions in chassis. On a class C, a diesel is pretty much only available on the smaller units (based on a Sprinter or ProMaster chassis) or on the very large units (based on medium/heavy duty truck chassis), while the middle-of-the-road units are practically all based on Ford or Chevy van chassis with gas engines. For class A motorhomes, diesels nearly always imply a rear engine pusher configuration, while gasoline powered ones practically always have a front engine setup. The chassis differences (front vs rear engine, etc.) do have a significant effect on how the motorhome behaves.
In terms of mileage or engine longevity, I'm not sure it makes much practical difference. Mileage is poor in any case, and with a bit of care the engine typically will outlast the rest of the motorhome whether it's gas or diesel.
(By the way, a motorhome is not an investment in the financial sense.)
