Alabama suggestions

Wheeler and Desoto as others have suggested are really nice in AL. Cloudland Canyon in GA is also really nice. Cloudland and Desoto are a ways from the I-65 corridor.

Another nice park is HoneyComb Creek on lake Guntersville, and if you like to fish, it is one of the best bass lakes in the country. Really nice waterfront sites. Also Guntersville is a great little town with some good restaurants, and little shops.

There is also a state park in Guntersville, that is nice, but has very few trees, a major tornado hit it a few years ago and destroyed the campground. In December no trees might be nice, in July trees are your friend :)

North AL, is a great area, and hope you enjoy your trip. You never know about December in AL, it may be 80, or it could be 20, you just have to roll the dice.

Tannehill State Park near Birmingham gets good comments.

try Point Mallard in Decatur Al. Joe Wheeler State Park in Rodgersville Al. Also A campground at The Space and Rocket Center in Huntsville but it only has 30 amp service. I suggest Point Mallard Campground. Not far off I-65 level and full hook-ups

Cloudland Canyon SP at Trenton, GA or Desoto SP at Fort Payne AL would be my picks in that area. Both are up on Lookout Mtn - Easy access...

Roy Ken

Henry Horton sp in tennessee about 50 miles south of nashville about 8 miles off of I 65 recently redone.
James Floyd sp in summerville ga or cloudland canyon in ga.
above parks are nice, only elec and water no sewer.
desota sp or wheeler sp in alabama cannot comment on these. if the al state parks are like the rest areas, they will be spotless, super clean.