If this is your first trip I would not make any reservations. As another poster mentioned the central part of Florida is nice especially the springs like Alexander, Juniper or Salt springs they are NFS CG's and are half price with the senior pass. Further south is Everglades NP and Big Cypress, I have not been there for 5 years or so but never had a problem without reservations if you get there mid week. I would also go to south Texas San Padre island there is a NFS CG with water and a dump station just off the beach. There is also free camping right on the beach. Further west is Quartzite AZ. which is all free camping and is interesting. You can decide after your first trip which area you you may want to return to for a longer stay then make reservations the following year.