Panther Creek State is about 45-50 miles or longer depending what campground you're at, and it's fun. You can easily get 20 miles out of it without looping the same trails. If you're on NC side of Smokies, well, you got Tsali, Fontana, DuPont, Bent Creek, and some lesser knowns.
Edit: Also, if you're going be near the Virgina Creeper, down interstate 81, there is a place called Warrior Path State Park. Darwin's Revenge is one sweet trail to rip through! :thumbsup: It might be dicey getting your RV into the trailhead, so if you're hauling an extra car you can leave RV at campground and take extra vehicle to trailhead(or ride on the road for a mile or so). The biking trails are on the other side of the park. So, you'll have the entire section of that park to yourself and fellow bikers.