If you have not done so, take a good at this website:
RV Park Reviews :: Home
It has a lot of information on campsites. You can use the search function of this site and RV.Net Open Roads Forum: Search
to get much more information.
GSMNP is fairly large so you need to decide what you would like to be nearby. Many people like Cades Cove Campground or Elkmont Campground.
I would recommend reservations if you plan to be there in the summertime.
The park's roads and trails get crowded. One way to beat the crowd is to get early starts on your trips. Gatlinburg and Pigeon Forge have popular attractions and are very crowded.
Enjoy your trip!