So I'm mostly looking at class C's and travel trailers.
Besides our own wish list of features, I am also considering our pets. I'm guessing by the time we go, we will be down to 3-4 parrots (several of our current 7 are elderly), 2 small dogs (one of our 3 is elderly), and the cat. In a motor coach, an ideal spot for bird cages would be in a bunk over the cab so there would also have to be a separate bedroom for us to sleep. If a travel trailer, a 2-bunk area would be a great spot to keep our cages - birds on top, dogs and cat on the bottom. (A toy hauler isn't out of the question to modify for cages/supplies either!) These are my considerations for our pets.
The class C's are nice because they aren't too big, and we all (people & critters) will be able to travel in comfort. That's the biggest advantage. But we'd need a toad; Pete and I are both big day trip people, and we would have to be able to get around wherever we are, and we wouldn't want to pack everything up every day that we want to go somewhere. I've seen some good workable class C floor plans that would accommodate our needs.
The travel trailers are nice for the reason I just mentioned: we need another vehicle to go places. That vehicle would most likely be a van, so that it can carry the travel cages when we're on the move. Which means 2 sets of cages for the birds; we could move the dogs' and cat's back and forth. But just one more thing to do when we move. OTOH, every time I see one with the bunks I realize how perfect that area would be for pets. And I also prefer the feel of a travel trailer inside.
As far as hauling a car with a motor home vs. hauling a travel trailer with a van, I don't know which would be more economical and which would be easier/more stable on the road. Those are things I still need to learn.
As far as features: I love to cook and bake, so while I understand a full-size kitchen is out of the question, I do want to have at least a little bit of counter space and a "real" oven. I also don't like the booth-type kitchen tables, whether they are U-shaped or not. I'd prefer a couple of swivel chairs with the kitchen table between. (I'm guessing we could modify?) I also don't care about having a couch, but would prefer chairs - preferably recliners. In a travel trailer, we could maybe have both, but if I have to choose, I'll take chairs over a couch. A decent-sized shower is another must-have feature; when I'm showering I want to be able to move, not bump into the walls or stick to the shower curtain every time I raise my arms. And we will need space for keyboards. I plan to only have 1 by that time, and Pete will probably have 1, plus he has a QChord...so we need a place to store these, and to play music. We have a music room in our home but our living room will have to double as a music room when we're on the road.
Internet is an absolute must. Whether we have a satellite or an unlimited air card or whatever...we have to have reliable internet. Satellite TV is optional, as there is plenty of content on the 'net to watch on our television, connected to a computer or cell phone. But technology will continue to evolve in the next few years, so this isn't something I will worry too much about right now.
As for the size, at the only camper show we've attended so far we decided that something in the 28-32' range would suit us well. It would depend more on how the space is laid out than on how much actual space there is. A 32' travel trailer feels really big; a 28' class C feels kind of tight. Slides make a BIG difference also - a travel trailer with slides can feel really spacious and homey!
Gas vs. deisel (class C)? No clue, haven't looked into it that much.
This describes the very early stages of what we like now. That may change in the next few years, with more experience on the road and with possibly different circumstances, but we have to start somewhere! :)