We were camping hosts at Big Bend Chisos Basin CG from Apr- Aug 2014.
He will not be taking that MH into the Chisos Mountains Basin. Road is narrow with 5 tight switchbacks. The rig won't fit anywhere in the CG.
All the other paved roads in Big Bend NP and on the way to the park are excellent. No serious grades or tight curves.
The only FHU camping within the park is the concessionaire run area at Rio Grande Village. It is a parking lot with utility hooks at the back of the rig.
Rio Grande Village has some sites that will hold that MH for dry camping. Generators can be run in part of the RGV campground. Cottonwood campground does not allow generators. A nice small CG - Cottonwood has a couple sites that would hold that rig.
Back roads for 4 wheel drive vehicles - some are excellent some the Rangers cannot travel at times. Check with a ranger / the visitor center each day about specific roads and conditions before setting out.
Busy season - Big Bend only bothers with campground reservations between Nov 15 and April 15. Winter holidays - Thanksgiving and Christmas are very busy. The busiest is spring break. second week of March to mid-April.