Campgrounds in Big Bend:
Campgrounds at Rio Grande Village (RGV) tend to be confusing.
There is a small concession operated full hook up CG run by the store/gas station. It is an asphalt parking lot and the sites are pretty cramped.
There is a large 100 campsite CG operated by the National Park. Dry camping only. Generators allowed on one area and a no generator area. About 60-70% of the sites are large enough for 35-40' rigs. A few will handle 45' rigs. This is a very nice CG with nice large cottonwood trees which provide
some shade.
NOTE: When reading reviews for RGV CG's be sure which CG is being reviewed, dry camping or FHU. I have seen reviews for the FHU CG in the dry CG review sections.
Cottonwood CG near Santa Elana Canyon is nice and usually quiet CG. Dry camping only and no generator use allowed. About an hour or more drive from the Headquarters at Panther Junction.
Primitive campsites. There are a few primitive campsites on the gravel roads. Some are in the website given earlier, which I think you looked at. In addition to the ones in the website, there are two more at "K-Bar" which are accessible by RV's up to 35-40'. Check out Hanold Draw as well. However no dogs or generator operation allowed in the Primitive campsites. Grapevine hills #2 & 3 are a few miles down a gravel road which may have washboard.
Chisos Basin CG. MH's over 24' and trailers over 20' not recommended. Partly because of sharp turns. But more so because the campsites are small, short and unlevel.
As stated earlier, September is still pretty hot in Big Bend. Either stay the the RGV FHU CG or stay outside the park in Study Butte/Terlingua, or out the N entrance at Stillwell.
Download the park newspaper here:
https://www.nps.gov/bibe/learn/news/newspaper.htm for lots of great info.