Get a Texas State Park Pass at your first state park for a discount on camping and for visiting other Texas state parks. Balmorhea is an oasis with a huge spring-fed swimming pool. We were there in mid-May 2007 following a wet winter and the bird life was incredible. We were there April 29, 2011, and it was like a different place. They were in a drought, had wildfires nearby, and it was 104 degrees with a 20 plus mph wind!
Also, we traveled the Natchez Trace from Natchez to near Nashville last fall. It's a great drive for RV's. We saw two large groups of Ford Model A's. It's a wonderful place to cruise in an old car, too! We stayed in the National Park Meriweather Lewis and Jeff Busby Campgrounds. They were good. They have no hookups, but have bathhouses and were free. It would be good to check to be sure they are open when you pass through.
We stayed at the Natchez State Park. It was good. We stayed at the LeFleurs State Park near Jackson, MS. It was okay. Both have water and electricity.