As much as I like the Rockport - Aransas Bay area, you said beach. That's a little further down the coast, and come March, whether it is warm or not (that part is iffy) the kids are going to be partying in the beach communities.
Same for east coast of Florida, whether it is warm or not (not always warm in Daytona), once the season starts anyplace along a beach that welcomes college students will be crowded. The 2015 season starts February 21st, runs through the end of April, with the big bulge being first week of March through second week of April. In Daytona, that overlaps with Speed Weeks which starts mid-February and finishes with Bike Week up through the middle of March. Actually, that also starts late January with the Rolex 24 hours.
To be more sure of being warm, I suggest Florida West Coast from Pasco county, south. For beach, that's Clearwater and south, then again off Bradenton and Sarasota, then off Fort Myers and down to Venice. There are not very many RV parks on the beach in that area, one I know of is Red Coconut in Fort Myers Beach.
Tom Test
Itasca Spirit 29B