We've wintered in our motorhome in the Rio Grande Valley of South Texas quite a few times since 2007. Usually around Mission or Mercedes and at different RV Resorts each winter. The Valley has everything you need for the winter stay as they have been hosting northerners for over a hundred years. The people are friendly and the prices are reasonable.
The "winter texans" are a vital part of their economy and are treated as such with huge welcoming parties thrown for them at the beginning of each season (other areas of the country view them as nuicances). There is even a migration route from and to Canada down to the RGV that roughly follows I-35.
It's one heck of a scene down there as hundreds of thousands of "winter texans" converge on the area each winter. And let me tell you, those "old people" party hard. They even have Cyclone Club events there, set up especially for the winter texans.
A couple of years we wintered in AZ, CA, NM moving from one RV Resort to another every couple of weeks. Spent a couple of weeks each time staying in Borrego Springs, CA while visiting my son who lives in Solana Beach, CA. That's the nice thing about wintering in an RV, you have lots of flexibility on where and how long that you want to stay. Plans can be changed on a whim. Some winters we are just too busy to get away, so no problem, we just keep the motorhome in storage here until spring.
Most RV spaces are rented by the season but some are rented by the month. Reservations have to been entered as early as possible, as repeat visitors get dibs on the spaces and the resorts fill up fast.
Our leased site one winter near Mission, TX that we ended up staying at for three months because it was so nice:
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