We went a couple of years ago. Dry camped at the CG in the Rio Grande Village location. There are some no generator sites, so keep that in mind. It's a nice setting, much nicer than the FHU's IMO. The FHU sites are at the same overall location, but are just back-in asphalt pads in the vicinity of the store. There are nice walks and hikes all over the place. You can pick up literature at the HQ and stores.
There is gas at RGV, and also at the main junction in the middle of the park, which is where the HQ is. There are small stores at 3 or 4 locations within the park.
There is a CG at the Chisos Basin. They warn against 26' vehicles but in reality you could get there. There is a big complex there... hotel, store, etc., and I'm sure they get supplied by trucks. Although I'm not sure it would be worth it. Just go with your toad to visit there if you are so inclined. The drive is more interesting than the destination, lol.
I think there is a small dry CG at the west end too. But all in all, I'd recommend RGV, the east end.
No real cell service that I can recall. Wifi at the stores.
It's worth seeing Terlingua too! I would stay at BJ's RV Park. It is a small park, doesn't look like much. But the couple that own and run it are super nice, and they get a nice friendly crowd. Their water supply is super nice reverse osmosis city water. They'll have campfires, and will gladly give you advice on where/what/how. Also, they'll come around and tell you "tonight we (a small group) are going to (XYZ) place for dinner. It's pizza night, and there will be a good C&W act. Wanna come along?" Terlingua is really a small place, you have to ask which restaurant you want to go to on a given night, because on the off nights a place will be dead. Still open maybe, but dead.
Whatever you do, unless you like plastic make believe, don't bother with seeing Lajitas to the west. They have a deluxe RV park, but it's all silly. Some rich person fell in love with the little old town there. Bought it and turned it into a fake western town and high end resort. It's even dumber in person than I'm making it sound. But if you head over to the State Park, you'll pass through Lajitas anyway. If you want natural flavor, Terlingua is the place. I've never been anywhere quite like it.