Whenever we travel, we stop at the various Flying J Truck Stops on the road. Their gas prices are usually among the lowest in the area. They have a great website which lists their prices at all their truck stops across the nation. You can check it out here as you get closer to your trip:
http://www.flyingj.com/fuel/gasoline_CF.cfm
I pulled up the prices at their truck stops along I-75 in the states you mentioned. Prices are for regular, plus and premium:
Walton, KY - Exit 171: 1.729, 1.819, 1.899
Knoxville, TN - (I40 & I75) Exit 369: 1.859, 1.949, 2.029
Resaca, GA - Exit 320: 1.769, 1.849, 1.939
Jackson, GA - Exit 201: 1.849, 1.939, 2.019
Lake Park, GA - Exit 2: 1.849, 1.929, 2.019
Dade City, FL - Exit 285: 1.919, 2.009, 2.089
Hope this helps you out. We always check their website before we leave so we know if there are any cheaper prices as we're travelling.