Caverns of Sonora near I-10 close to the town of Sonora. It is on a private ranch and has a small RV park. It is nothing fancy, but handy for seeing the caverns. They contain some pretty spectacular formations.
I would recommend Balmorhea State Park off I-10 in West Texas, but it is closed until Spring 2020 for renovation. It is the world's largest artesian spring fed swimming pool & open year round. Maybe on a later trip?
Also a short stop at the Texas Canyon Rest Area on I-10 near Willcox, AZ is worthwhile. You may already know about this. When we stopped there a couple of years ago, some folks in a small Class A were so excited about getting out and checking out the view that they forgot to set the parking brake in the rig. It rolled down the entrance ramp back to the highway and crashed into the ditch. They were being pulled out by a big wrecker and had a smashed windshield at a minimum.