If you want my advice about camping in Texas, DON’T. Just kiddng ;). I would just recommend not going in the middle of summer.
Besides the heat, we had a great time camping here in Terlingua, TX. Here is a link to the hipcamp listing:
Only 7 minutes away from the entrance to Big Bend National Park, this was the perfect spot to camp for the night. We had a great time cooking dinner with the stunning mountain views in the background, taking a sunset walk down to the river, hanging out in the common area (with WiFi to work!), and star-gazing after the sun went down.