First, all sites in the park are first come, first served. No reservations. There are no hookups except at Rio Grande Village.
There are 3 campgrounds in the park. The one at the Basin is limited by the signs on the road to 20-foot trailers (though I have seen bigger trailers up there). I would take my 22' International up there, but nothing bigger.
Cottonwood campground near the mouth of Santa Eleana Canyon is down a looooooong road. If you get there and it is full, it's a looooooong way back. You can check at HQ for site availability, but it may fill before you drive the 25 miles to get there.
The campground at Rio Grande Village has two parts. There is a concrete parking lot with trailers wall to wall that has hookups. There is also a no-hookup overflow area that most always has space except maybe in spring break. I hated the area with hookups since I don't like to be wall to wall when I'm out with the trailer.
All that said, I stay just outside the west entrance at a campground in Study Butte. Full hookups, not much shade, decent restrooms, fair restaurant. It is about as close to things as staying inside the park.
Be prepared for a lot of driving. The place is BIG.
There are also a couple of smaller RV parks in Terlingua. Pretty shoddy, as I remember.
John W. Irwin
2018 Interstate GT, "Sabre-Dog V"