First, I hope everything works out very well for you and you have a fantastic time on your upcoming trip.
The choice of camper is still your decision. As I am not an "A" or a "C" owner, I won't give any recommendations which is the better choice. I do know, they both have their advantages and disadvantages. And as far as a rental goes, you'll probably find more availability renting a "C". Check Cruise America for a start. And be prepared to fork over a good chunk of change for the rental. You may want to re-think buying one of your own, and may find a used one more affordable. Just a suggestion here ... check into the costs.
I will respond to your intended travel. As a general rule, the "general" conscious on these forums is an average speed of 50 mph for your entire trip. If you are planning at driving 80 mph, and traveling 80 miles in that hour as a basis for the distance you are planning on traveling, well.... it just doesn't work out that way.
If you are planning on driving 300 miles, you should plan 6 hours @ 50 mph. If you think you can average that 300 miles in 4 hours and 20 minutes @ 70 mph, that just probably will not happen. Between fuel stops, rest breaks, sightseeing along the way, traffic, potential road problems, detours, and any number of things, you will be traveling more realistically, averaging 50 miles and hour.
If you plan your trip at 50 miles per hour, your distances will be more realistic. One thing you do not want to do, even in a motorhome, with the advantage of your passengers being able to move around, traveling, you and everyone else, will get worn out and you'll need a day on your travels for some down time and rest.
If you are planning on zooming to (lets say, Las Vegas) and then you are there for a day, and you run as fast as you can to see it all, and then the next day you hit the road again, you're going to be exhausted, very quickly, you'll not be enjoying anything. The entire trip will be remember as a blur. Not to mention the very real possibility of everyone getting on everyone elses nerves being trapped inside a 35 foot box without space escape each other for a while. Once tempers begin to flair, it's hard to regroup and make the rest of the trip a fun experience after that.
If you travel 2 days, then I suggest the 3rd you park it and rest, have fun, explore an area, but the biggest thing is to get some rest.
This may change to the total distance you can travel, but it's really better to travel half the distance and enjoy what you experience twice as much!
That's my only suggestion.... Anticipate your daily stops based on an average of 50 mph. If leaving at 8:00 am and driving till 4:00 in the after noon so you can find a place to park for the night, get your utilities hooked up, and get something to eat, an 8 hour driving day is pretty realistic, that would translate to 400 miles a day.
Good luck, and enjoy!