I'm in Big Bend Ranch State Park now, and just left Big Bend National Park.
The only campground in the National Park really suitable for RV's larger than 20 feet or so is one of two at Rio Grande Village. The state campground has a dump station and a water fill station at the dump station. The sites have no utilities at all, but many are large enough for large RV's and are in a shaded area. Some sites do not allow generator use at all, but most allow 8am to 8pm.
There is a private, full hookup campground at Rio Grande Village. It is essentially a large parking lot - but it's a parking lot in a pretty area.
The Basin area can not handle large RV's (nor can the road you take to get there - it is narrow, and has very very tight turns). At Castolon, there is Cottonwood Campground. I've never stayed there in an RV, but the sites are generally small. I do not think there is a dump, and of course no utilities at the sites.
At Big Bend Ranch State Park, there are really no sites for larger RV's; there are a few that might work for a pickup camper. There are several private RV parks nearby; I am at Maverick Ranch in Lajitas. It's nice, but kind of pricey - $39 a night.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch