Take a stroller for younger kids and ride MARTA from lot near Stone Mountain in to Aquarium, CNN, Centennial Park all in area of single stop. Grant Park (home of Zoo Atlanta and Cylorama are a different stop. Both on the east/west line from Stone Mountain into town.
Absolutely agree with all who say do NOT drive RV in to the attractions. Almost all parking is pay lots, and will get you no closer than MARTA. I will not drive into downtown in my pickup if I can get out of it, as it is too large for narrow crowded streets.