It doesn't sound like an RV is for you. It's mostly a lifestyle (or travel style) choice and unless you've done it before and find that you really like it, a trip of this magnitude is not the place to try it out, regardless of what others (who love that mode of travel) might say. If you're traveling for the scenery and want to stop at motels and ranches, then what you want is a vehicle that gets pretty good mileage and has good visibility. A convertible certainly fills that bill, but will cost a bit more than a standard sedan. By the same token a 4x4 (light truck) or 4WD/AWD car are not really going to help you on this kind of trip. You can get pretty much anywhere you want to go in a normal sedan and besides, if you read the fine print on any rental contract for a rugged SUV it will almost invariably prohibit taking the vehicle off-road in any event. Just get a comfortable car that will get you where you want to go in some semblance of style and ease. The rest is all just window dressing and usually not worth it.
You mentioned in an earlier post that you were thinking about staying at ranches and such. That is also possible and such accommodations range from B&Bs similar to the Irish Farmhouse accommodations to Dude Ranch Resorts that will cost you several hundred dollars a day but include horseback rides, lectures and all meals. Everybody's tastes are a little different, so search the web using the terms "dude ranch", "bed and breakfast", "vacation rental", and the name of any town or area where you think you may want to stay and see what pops up.
By all means get a map of the U.S., or better yet just the region you'll be traveling in and start marking it up. Not only will it give you a clearer picture of where you're going, the distances involved, and the other sites that are nearby, but it will also serve as a great reminder of your trip years from now. And personally, I would think that Denver is a bit far afield for a 2 week trip through the American Southwest. I'd limit myself to California, Nevada, Utah and Arizona. You can't even do just those 4 states justice in two weeks. Trying to do more would just spread yourself too thin.
AZBuck