Try James Sloppy Floyd SP, I have heard it is nice. I have been to Cloudland Canyon and it is beautiful in the fall, haven't been any other time of the year. Red top Mountain SP on lake allatoona is your best bet if you want to stay on that side of ATL. Problem with GA SP's is that you cannot reserve a specific site, first come only. That is difficult if you have a big rig and want to go to the parks. There are some really nice COE parks on Lake Lanier if you want to go to the eastern side of ATL. Unfortunately, there is not much water in Lanier right now.
PM me if you change your mind and want to come to the NE side of ATL, I have been to most of the parks on that side.