I like to stay at Stillwell Ranch, It is just South of Marathon on the way to Big Bend. They have a web site. If you go there be sure a try one of Nan's burritos, if she is still making them. There is a full hook-up parking lot by the store in Rio Grande Village as well as dry camping at/on the river just East of the store and Cotton Wood camp ground. I believe the two dry camp areas are "no generators". On the other side of the Park is Terlingua and there is a full hook up park there. As you are probably aware, things are in limited supply out there, stock up on the way in. Wish I were going.
