You'll be limiting yourself a lot by buying a 40' fifth wheel. Most people who buy large RVs find that it's best to park in a commercial park with full hookups and take day trips. Getting to some of the coolest places involves driving on curvy mountain road which probably wouldn't be fun towing a big fifth wheel.
If you can, take a trip to the Great Smoky Mountains National Park and check out the sites. Here's link that will give you an idea of the recommended size limits -
http://www.nps.gov/grsm/planyourvisit/frontcountry-camping.htm Also take a look at this site -
http://www.hitchitch.com/links.html Most of the bloggers are fulltimers with all types of RVs and lifestyles but if you search through them you'll find the ones who lead active lives which will give you a better idea of what to expect from public parks and boondocking locations.
