I know you said boondocking, but High Falls State park is at exit 198 on I-75, between Locust Grove and Forsyth, and is a good place to stay. $32 night, $224 a week.
Just west of Macon is Lake Tobesofkee, and it has three parks operated by Bibb County, and two of them have campsites for RV's. The lake is quite large and easy to find on Google maps.
Camping | Lake Tobesofkee
Rates | Lake Tobesofkee
There appears to be a somewhat remote camping area that has good roads up to it, but is not mentioned in the Bibb County website.
Google maps Lost Lake Camping area, Lake Tobesofkee
The two parks with camping sites.
Google Maps, Arrowhead Park campground
Google Maps, Claystone Park camping area which is only 2½ miles east of exit 5 of I-475, straight shot, easy access.
Me personally, I would consider parking the motorhome and getting a rental car cheap as this is a large area you are wanting to look at.
Charles