These type of topics always get people to defend their choices so don't expect unbiased comments. We investigated the Rio Grand Valley and decided it wasn't for us. We decided to go to Port Aransas which is on the barrier island off of Corpus Christi. We stayed at Pioneer RV Resort
and enjoyed it enough that we are going back this winter. Port A (as it is called) is very low key but it is only ~30 minutes from Corpus which is large enough to have good shopping, restaurants, etc. We could walk from our site over the dunes on a boardwalk to the beach. Pioneer isn't an over-55 place, but during the winter that's pretty much all the people who are there.
Port A is probably a few degrees colder than the Rio Grand Valley, but daytime temps were usually in the 60's and the worst we had at night was a couple of low-40's.
Another TX alternative would be the TX Hill Country near Fredericksburg but that is another ~10 degrees colder and that's more than we want.