We use BEST HOLIDAY TRAV-L PARK in Chattanooga, TN. It is actually located just inside GA, right off I-75. Nice park with pull thrus. We come from Peoria, IL area, and its about 1/2 way. Might work out for you as well. I think that's about 600 miles from Detroit Rock City....which is about the same for us. We leave early (about 630a...wish it was earlier!). We stop 1/2 way to our overnite at a reststop for lunch, and usually get there by 5pm. From Chattanooga, its about 580 miles to Ft. Wilderness.
Website is travelparkchattanooga.com.
NOTE: Raccoon Mtn Campground is also there, but really out of your way as its on the EAST side of Chattanooga, so you'd be going out of your way to get there. Plus, we found it small, harder to pull in/out.