Most RV Parks in the RGV are located betweeb Mission and San Benito. If you are going to spend your time looking at RV Parks for the long haul, then I suggest a mid valley location. If you pick a location at either end of the valley a round trip to the other end and back will be 100 miles.
The cities of Donna, Weslaco and Mercedes are mid valley. Victoria Palms is located right off the Expressway which is the main east west road in the RGV. It is on the Donna/Weslaco border. Another Good choice wood be Llano Grande in Mercedes. Both are high end RV Parks.
A point to note. Some of the roads in the valley are being renummbered. Expressway 83 is now Interstate 2 and US 281 and US 77 are being renumbered as Interstate 69C and Interstate 69E. I-69 will have three branches in south Texas. The main one goes to Laredo, the Cental one to Pharr and the Eastern one to Harlingen.