I'm not sure of the exact mileage, but Cades Cove up in the Smoky Mountain National Park is amazing and offers loads of scenic vistas, 99% of them right from your van window. It's a loop drive and you'll be sure to see lots of deer, bear, turkey and other wildlife.
I know anytime we're in that area, its a must see destination for us.
There is also the Blue Ridge Parkway. Most of the scenic overlooks are beautiful right from the van and depending on time of day and season, you have a host of scenics to choose from.
I know most of the major national parks have ramps, boardwalks and are very handicapped accessable. We're in Yellowstone right now, and except for the back country hikes, you could see everyone of the geyser fields and scenic vistas from a wheelchair. Probably one of the best parks I've been to when it comes to making sure you have accomodations
Good Luck, and good for you that you are still shooting