re fall visiting to Big Bend. It probably won't cool off there until mid to late Oct. But check on the date for the annual Chili cookoff in Terlingua, because the area will be busier than usual at that time. It's usually in NOV.
The FUH in the park is literally a paved parking lot with hookups. Rio Grande RV park. Next door is the Rio Grande campground, with lots of shade and water and a dump station. There's a little store inbetween the two campgrounds. Make a reservation NOW if you really want elec. hookups.
In Terlingua, we have stayed at the Big Bend RV park and the Study Butte RV park.
Neither is fancy...both provide the basics and are close to the park's west gate.
Again, if you can, make reservations.
On the way to the park, if you use highway 90 out of San Antonio, there are lots of rv parks in Del Rio, and a state park campground near Comfort TX. And at Marathon, one of the gateways to Big Bend, there is a decent rv park in town and a good restaurant at the hotel.