I think the best advice you could get would be to rent a unit for a trip.
It is a very large investment, even for a used unit, if you have no previous experience with an RV.....especially to consider going Full-Time in it.
There are also lots of questions about your own particular lifestyle and desires that will help you with your search.
First of course, is probably the amount of money that you will have to invest in the unit....as this will narrow down the playing field considerably, depending on the number.
Where do you want to travel, and where do you plan to stay? If you want to go to places that may not handle a large "A", you may want to look for a smaller unit, or even a "C" to allow you to stay some of the places that an "A" may be to large for. On the same note, if you plan to stay in some of the nicer (and more expensive) RV resorts, an older RV may not be allowed.
How many are there going to be using the RV? If it's just you and a spouse, that should be no problem, but if you have a bunch of kids, the sleeping arrangements may become an issue with the "A"
Diesel or gasser? Not really an issue, but it will be something that you may want to consider. The diesel will give you more power if you are climbing the mountains out west.....but you will pay more for a diesel than a gasser, both initially and at the pump. Yes, the diesel will typically get better mileage, but when fuel is $0.50 more than gas, I don't know if the savings is actually there?
Have you ever driven a large RV or truck? It isn't hard, and practice makes perfect, but if you just spent thousands, and then you find that you are afraid to drive it, you wasted a bunch of money......another reason to rent one first.:)
I'm sure others will chime in with other suggestions. I am not trying to turn you away from a wonderful experience, but you do need to understand as much as you can. There is lots to learn on this site, as well as on-line or in magazines. Learn as much as you can, and learn which questions to ask.
Good luck, and welcome to the Forum:)