My opinion -
With the vintage you're looking at prior owner care and feeding has a much bigger roll that who the original builder was. Starting with a quality builder will help but a $500,000 rock start coach will rot just as fast as a $50,000 rig if not cared for properly. There's plenty of brand loyalty but it one builder better than the other? Hard to say. We've enjoyed every minute in our bargain rig and plan to keep it for many more years.
If you can fit a diesel into your budget I would go that direction. Considering how far you plan to go to your summer destination and how long you plan to stay a diesel might be a bit more comfortable.
Concerning mileage you want to see some. Say more than 3000 a year but less than 10,000. Same with time on the generator. Should be about an hour for each month of the it's life. If you go gas try and stay with fuel injected engines rather than carbonator or throttle body. The older systems worked well but it can be difficult to find someone who knows how to setup a carbonator properly but you will be stepping back into the 1990's to find a rig with such. The various power trains available are somewhat different (Ford, GM, CAT, Cummins,,,) but each will get you to where you want to be. Again each has pros and cons. One over the other is a Coke - Pepsi question.
Just think of a class A in the same light as your fifth wheel and truck, combined. Chassis, tires care and such for a class A isn't much different and you had pretty much all the same house systems as a class A. Good luck with your search and once you narrow down your focus start asking questions again.