Not sure what amenities you are looking for, how much you want to spend, what kind of activities you like, or when you want to go; but there are a lot of snowbird campgrounds in the Rio Grande Valley. We stayed at a nice place called Bentsen Palm Village in Mission, TX last March. It is right next to Bentsen-Rio Grande Valley State Park where there is a lot of birding available. Also, it is good for bike riding and hiking.
There are flea markets in the area and there is also the Santa Ana National Wildlife Refuge for hiking and birding. It's a short drive to South Padre where you can take a dolphin boat tour and where you could also do some fishing. Or you can take a short trip across the border if you want.
The only problem is, we tried to make reservations for Bentsen Palm Village when we were there last March for Jan. and Feb. of 2008, but they were already booked. However, they said they may get cancellations.
There are many other RV parks in this general area you could try. Some of them are bound to have openings. Google "Rio Grande Valley" and see is they have any campground links. Also try a search on rvparkreviews.com
. Look for towns like Mission, McAllen, Pharr, Weslaco, Donna, Mercedes, and Harlingen.
We were reasonably warm most of March with temperatures generally in the 60s and 70s, although it did get cool (30s and 40s) some nights. This area of Texas, however, is very windy. We had 20 to 40 mph wind almost the whole month of March. Even at night, we could hear the slideout toppers flapping. Another couple who has a blog that we follow spent Jan. through most of March in the Rio Grande Valley and they complained about the wind as well.
Best wishes in your search.