The great spots to staywarm are in the Rio Grande Valley. The great spots for beach, ocean, water, fishing are of the Gulf Coast, from Rockport to South Padre Island, including Port Aransas, Mustang Island, and Corpus Christi. Or even Houston to Galveston Island if you are more tolerant of the more frequent colder days.
The great spots for Texas and Tex Mex culture are San Antonio and the Hill Country inside a triangle roughly defined by Austin, San Antonio, and Fredericksburg. Also warm enough for me, but not for the folks who go all the way to the tip of Texas and still huddle inside on the cooler days, because no place there is as warm as South Florida for as many winter days.
I also like the western High Plains and Texas Rockies: Big Bend, Fort Davis, Midland/Odessa and even over toward El Paso.
Big place, Texas, size of France and almost as varied. I suggest for first trip, you get the state tourist guide, all the scenic trail and historic trail maps, RV campgrounds guide (all from tourist department or highway department) and plan to spend the winter moving around, find your own favorite places.
My not to miss, and favorite for revisting, is San Antonio. There are a couple of RV parks on mass transit routes, one south on the Mission Trail, one out toward the airport. Both in urban setting, bothers some people, but easy to get around on the buses. SA is also a good central location for getting into the Hill Country towns, many of my Hill Country visits were from daughters home on north side, the two years she was there. But SA is a big city (#2 in Texas, in top 10 U.S.), just feels a lot smaller than it is.
If looking for rural, think about Fredericksburg or Johnson City, small ciies with rural surrounds.
For beach, Corpus Christi for urban amenities, Port Aransas or Rockport for small town. Well, Rockport is not really on the beach, but it is on the shore. Not go to the beach weather, unless hardy. Gulf Coast of Texas the beaches in November-February will be a lot like Lake Michigan beaches in October or April.
You can get most of the info on the state at welcome centers on US-69/75, I-35, and I-44 but better to go to website, order the free tourist guides, event guides, trail maps, and do some planning before you go. 30 years in Oklahoma, at least 20 one to three week road trips into Texas, all different. Use the first winter to explore, rather than trying to find a place to settle in for 2-3 months.
