My first question is - are you planning on staying at Stone Mountain for a visit, or only an overninght stop?
If it is only for an overnight stop it is well out of the way for your trip. Just north of Atlanta there are a couple of beautiful state parks and COE campgrounds that are only a couple of minutes off I-75.
Now, with that said when we go we stop in Ocala at Silver River State Park. Its about 15 - 20 minutes off I-75 but it's a beautiful campground/park. We generally stay there a couple of days to get into the vacation mood then get up early the day we are scheduled to arrive at FW and drive in early.