Renting first is the best option, as you'd learn on the RV forums. At least that'll narrow it down for you as to gas vs. diesel (nearly all will tell you diesel if you can afford it....even the gas owners will tell you diesel). Also, a class A, B or C will narrow down for you pretty quickly by renting.
Many newbies believe that the class C would be easier to drive (the van or truck cab) but in reality, veterans will tell you that the class A is much easier. My short time RVing it seems like I see many more women driving the class A than the class C. The class A does turn sharper and you're further forward in the vehicle than in a class c and most class A's will have back up cameras on both outside mirrors.
Tiffan was mentioned above. We actually met Bob Tiffan and his wife at the big Orange Country RV Show last year.....in a beautiful Tiffan Bus. The Bus is not really a bus, it's their top of the line motorhome and as soon as you step in one, you'll know it's their top of the line. The Red and the Allegra are also very nice. Many other brands that are equally nice and even nicer.
You can rent just about any kind of RV with the exception of a 5th wheel. If you're going to go with a trailer or 5th wheel I'd suggest you get a 1 ton. The 3/4 ton really limits you to the small 5th wheels.
What ever you get now, you'll probably change in a few years so buy used first! RVs are not like cars/trucks.....people only use them a few weeks a year (unless they're full timing) and so you can find many that are several years old that look new. Many great deals out there on used no more than 5 yrs. old. Let somebody else take the hit on depreciation. The high end class A that are ten years old or older, diesels, are a real value and many with low mileage. 50,000-75,000 miles is nothing on a diesel pusher.
Here is a link to the largest RV consignment dealer in the nation. They only do consignments, no new sales so you will not be pressured into an up sale. I also use it to get a general idea of the current value of used RVs as well as NADA Guide. Remember, these are consignments and only "asking" price and so sale price will usually be lower....
http://www.pplmotorhomes.com/And here's a link to RV Consumer Group. They rate all makes and models of RVs every year. It's web based. They don't have the ability to personally test and review each make/model every year personally. They do it through surveys sent to actual owners so you're getting a real world, hands on ownership compilation of reviews. It's a bit pricey but I thought it was worth it. I was prepared to buy a 5th wheel that "looked" great. After getting the RV Consumer Group list of reviews for 5th wheels I learned that model and even that make in general was rated pretty low and I instead went with a make that wasn't even on my radar.....
http://www.rv.org/