I have not done it and I can't speak for the laws in every state, but I would never get anything bigger than a 40' myself only because you add a lot of restrictions. In California it requires a special drivers license and then there are many road restrictions. For example most of Highway 1 has a 40 foot max limit. And the many different cities have ordinances that place restrictions on vehicles over 40 feet and where they can and can't drive.
I know quite a few people on here have them and love them and that's great for them. I just don't want the extra hassle for the extra five feet. Not to mention all the things you brought up about that many more camp grounds you can't get into.