WE have been wintering in Mission, Texas, for several years. You won't have a problem finding a place. It's popular here, but there are a lot of parks and all price ranges as well. It's a reasonably inexpensive place to stay. There is a national birding center located here in what used to be Bentsen state park. Also a butterfly center which is pretty cool. Lots of parks nearby. . . Mission West comes to mind as being a good park for transients, but there are many others. You can always get a spot there and they have lots of activities. Come stay for a few days, scout around at other parks in the area. We are still 90 miles from Brownsville, so there are hundreds of parks from here to the ocean.
Dale