Keep in mind that for most Texas state parks, you can reserve a site, but not a particular site. Choice of sites is first come, first served, so arriving ahead of a weekend is always a good idea in the more heavily used parks. Talk to the rangers at the park ahead of time about the number of sites that can handle the length of your rig. Tell them when you check in as well and they will try their best to get you in an appropriate length site. As others have said, some of the older parks may not have as many longer sites.