We LOVE Texas, but HATE the border area of the Rio Grande Valley. We found it dirty, congested, and no different than "camping" within the boundaries of any great big city. (And now, at this point, the "RGV Varsity Cheerleading Squad" will jump in to tell you how wonderful it is). :B
If some occasional cool nights don't bother you, try the Hill Country west of San Antonio (Kerrville, Fredericksburg, etc.). Beautiful, hilly country with lots of rivers and streams......very reminiscent of Tennessee. If you like it a bit warmer, and enjoy the beach, then check out Rockport, Port Aransas, and Mustang Island areas. You can camp right on the beach on Mustang Island if you like boondocking.
We did one month each in several areas, and finally settled on Rockport, but there are so many beautiful and interesting areas in Texas to explore.