Do what jetcare suggested. Do your research. Make a list of places that you want to visit and pick an RV park for the first few weeks in the RGV. Ask other RVers in your RV park for suggestions.
I would suggest going to a park that is mid valley and close to the Expressway (I-2 / US 83). This way you can get to and from prospective RV Parks easily. The towns of Donna, Alamo, Pharr and San Juan are mid valley. Have fun exploring different locations and RV Parks. You will not need a reservation for a place to stay...... a reservation might be needed for a specific type of site in a specific RV park......... but you can't make one until you see the site. Once you pick an RV park and site for the rest of the winter you can walk to your hearts content.