385 and 118 are both well kept roads. Top off your fuel at Marathon if you can because the prices are much higher in the park.
it will be HOT down there this time of year.
Rio Grande Village campground has a lot of shade which will help. There is a small, parking lot campground with electricity if you can find an empty site.
Or you can go into Terlingua and stay at the Big Bend Motel and RV resort except that it isn't a resort...but it isn't bad and has electricity and isn't very far from the Park's entrance gate.