There's not much out there as I recall. You may run up on a place or two as you drive along. I usually stay in the Dallas Ft worth area at Treetops RV located on the south side of Arlington. Treetops RV Resort in Texas | Carefree RV Resort
This is really a nice spot close to restaurants. From their location its not all that far to I-45 south.
In Galveston our favorite place to stay is Jamaica Beech RV Photos
In the front row spaces you can actually get a view of the beach and gulf. Real clean and great pool and miniature golf.