I don't know about restrictions, but Big Bend is one of our favorite places to visit.
We have stayed at an RV park at the junction of Hwys 118 and 170...I think they are listed with Big Bend Resort and Adventures. 170 goes to Terlingua. On hwy 118, south of the RV park is Cottonwood General Store...you can find almost all grocery items.
This RV park is a short drive from the entrance to Big Bend park.
The last time we went, we took a little trip to Boquillas, Mexico...crossed the Rio Grande in boats, rode donkeys the 1 mile to the village. We ate lunch at one of the 2 restaurants...great Mexican food, friendly people selling handmade trinkets. Of course, passports are required. Really great experience.