Some good advice here (and some that's IMHO so-so).
I wouldn't be too scared off with the size of a motorhome for driving. The length, in particular, is probably the least important dimension for how the driving goes; the back end will dutifully follow the front, whether it's 20 feet or 40 feet away. The width and height of a motorhome do take some getting used to, as of course does the weight and general lack of agility, but that's more or less true for most any motorhome.
I have a 32' class C. I don't full time, not by a long shot. So far, I've generally averaged between 7.5 and 8 mpg, probably closer to 7.5 mpg. Tires ran about $1000 to replace six. Larger tires such as many class A's have are of course a good bit more expensive. The tire companies generally recommend professional annual inspection (or just replacement) starting at around 6-7 years, regardless of mileage, and replacement at 10 years. (I'm not sure where ng2951 got three years from.)
Fuel expenses of course vary with how frequently and how far you move. Campground fees vary with where you stay and what kind of amenities they have. $2000 a month certainly sounds doable to me offhand, though I haven't sat down to really crunch the numbers.
How big a unit you need to comfortably live out of really varies with how much stuff you "need" and how much space you "need." I would probably look for something with at least a permanent bed, as making and unmaking a dinette or a couch every night would get old pretty quickly.
