For us it is 3 hours to Detroit on ON 401 then I-75 all the rest of the way. Just be prepared to lay low for a day or two somewhere if the weather turns nasty. The highest elevation is the first 15 miles or so into Tennessee. Two years ago we spent two nights at the KOA at Chattanooga South while Atlanta tried to deal with 2.6" of snow. In Michigan and Ontario we call that Spring.
When we used to do it in early January our first stop was Kentucky Horse Park at Lexington. No water or sewer but the electricity is on and the washrooms and showers are open and heated. There is water available at the dump station. Our second night was the KOA at Forsyth, GA which put us south of Atlanta for the next morning. That is also where we de-winterized. Since this is your first trip I would suggest a hotel until you get into Georgia.
As for Atlanta go straight through. Avoid rush hours, always keep one lane over from the right as the right lane sometimes becomes an exit, keep to the speed limit, leave room to stop, and most importantly remember that this is not the time or place to start a discussion with your partner about your in-laws coming to visit. Just take a deep breath and stay calm, Atlanta is not that tough.