Don't know if this will help but this site mentions motorcoach parking at the Aquarium along with the school buses. Georgia Aquarium | Directions
There is also a telephone number for the Aquarium - you might give them a call to see if they have suggestions. 404.581.4000
We toured it a couple of years ago - left the motorhome at the RV park and drove the small car into town. The area fills up early so I'd suggest you plan on being in that area early. There were some small parking lots in the area where you might be able to park.