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.