Reading what you wrote I think you'll enjoy Texas once you get onto it. Just something to remember, nobody in Texas goes to the RGV for a vacation. Most snowbirds go there because they have permanent places, or maybe in the beginning to get the cheaper price on lot rent based on a long length of stay. For the Texas newbie, that is a very bad way to save money.
Texas is a big state with lots of great camping spots/parks/towns. Texas is so big you can view it as upper, middle, and lower Texas and move with the temps during the winter months. Each of those sections have their own unique flavor, you only have to slow down to enjoy them. Spend a month in the Rockport/Fulton area, another in Hill Country, A few weeks up around the border enjoying the wild flowers in spring. Galveston Island is nice place to kick back, maybe hop on a cruise ship even for a week in the western Caribbean. Then there are all the places that don't get talked about on the internet, but kept secret.
On your drive south from Iowa try to hit Fort Worth on a Sunday morning. Take hwy 77 as others have suggested. Winstar Casino and RV park on the OK/TX border is a great spot to regroup and get things all washed up. Lots of nice COE parks in Texas, with a golden age passport they are very cheap even. Go with the idea of checking out Texas, and not just the RGV and/or Rockport.