Re: speed
I used to tow a 25' fifth wheel with a 2500HD GMC pickup, and had no trouble going 75 with that setup (weather and roads permitting, of course). Not up hills of course, and the gas mileage was poor. When gas went up the first time, I found much better gas mileage and less stress at 55, but that was preferance, not a requirement. My class A is limited to 65, but that is mostly due to the tires, I think, which are only rated to 65.
Probably many RVs (with the proper tires) will go that fast if you need to. Perhaps a more important criteria is if they will 'fit' where you want to go. A fifth wheel is difficult to back up and doesn't like dips or bumps. Long motorhomes back up easier but are even more likely to 'hang up' on rough terrain and don't turn well.
If you only weekend camp and don't need to carry a lot of stuff, perhaps a class B (van camper) would be a good choice (good for getting from here to there, perhaps a bit cramped when you get there). Relatively good gas mileage and will fit most anywhere. There are many available, or you can get one built to your design from Sportsmobile, even 4WD. If you need more space, the perhaps one of the smaller class C (van frame) would do.