Tip #1. Make sure the air conditioning is working in your car. If your car does not have air conditioning, rent one that does. Don't even think you can make it with the windows down. It's so humid that your sweat will NOT evaporate. And when you have to roll up the windows during rain, the condensation will fog up inside the windows. You will feel like you're going to suffocate. The cooling of the air conditioning is good, but its main function is to remove humidity from the air. Southern air is thick like a blanket in July and August.
Tip #2. Stay in cabins, not tents. Once you arrive as far south as Kentucky, give up the idea of tent camping and find state parks with cabins to rent. My friend from Kentucky says it's too hot to sleep and the mosquitoes will also keep you awake all night. (Hope to find links for state parks to include here.)
Tip #3. Drink lots of water. When sightseeing in the south, always carry a bottle of water with you. Heat exhaustion will ruin your day, and staying hydrated will help prevent it. (Although I've lived in the south for most of my life, I get heat exhaustion very easily and it's made me pretty sick once or twice.)
Tip #4. Sunburn. People from northern climates often have an idea that the sun is their friend. This is NOT true in the southern United States. If you are sightseeing outdoors, wear a hat with a wide brim, sunscreen, loose, light-colored clothing. Clothes that cover your shoulders and arms to the elbow, as well as legs to the knee, will be cooler than clothes that expose more skin. In a very short time, it's possible to get a sunburn that will make you run a fever. Even overcast (cloudy) days can give you a bad sunburn.
Tip #5. Bring a light sweater, jacket, or long-sleeved shirt. Paradoxically, this is a warning about being cold in the Deep South summer. Restaurants and dance halls with air conditioning may be cold. The dancers don't want to sweat too much, and a high level of air conditioning is needed to cool the kitchens enough for people to work there. If you are dancing, you won't be cold; but if you are just sitting, and not used to air conditioning, you might need your sweater.