We were hosts at the Chisos Basin CG from Apr to Aug 2014.
There are two spots for a rig the size of yours - the two hosts spots. They will be occupied.
It will be 90-95 for highs most days in the Basin, 100-110 at Rio Grande Village. The nights at the Basin will be down into the high 60s, the low 90s for RGV. You will definitely need the AC. (We slept only 3 nights in 107 days with the AC on.)
As mentioned - count on 45 min to one hour for the drive from RGV to the Lodge, about the same from Study Butte/ Terlingua.
Yes the road to the Basin is 'posted' for RVs 24 ft long, trailers 20 ft long max. I've seen Class C's at 29 feet long in the Basin CG, and I've seen 40' toy haulers in the Lodge parking lot (They were smart enough to not try to bring them into the campground, because they would have had to back uphill to get out. No turnaround room).
Tour bus drivers head into the mountains for the Lodge without any special provisions. The big 40's longer busses.
I do NOT recommend taking a 'too big' rig into the mountains/ Basin. If traffic is heavy, you might have to stop still on a 14% up or down grade and restart to make the switchback hairpins.
Google Earth will give you a good view of what the road is like. Just remember the last 1/2 mile before Panther Pass, including the uphill switchback climbs a bit over 700 feet in elevation. Followed quickly by four blind 180 turns descending almost 500 feet in 1/3 mile. Reverse on the way out.
Big Bend is a fantastic place. Great variety in a mountain, a desert and a river ecology. Dozens of very interesting stopping points along the roads.
Full-Time 2014 - ????
โNot all who wander are lost.โ
"You were supposed to turn back at the last street."
2012 Ram 2500 Mega Cab
2014 Flagstaff 832IKBS TT