It's a huge region, as you probably already know. If you camp close to Marfa, you'll be far from everything else. Although with fast speed limits and open roads, things seem closer. Nearest I've come to boondocking is a couple of the "roadside" campsites in Big Bend NP. No electric or water, but lemme tell ya, they're in the boonies! Davis Mts State Park might be the closest I've camped to Marfa and it's a lovely state park with immense landscape views and nice campground with a restaurant in the old CCC lodge.
To the south there's Big Bend Ranch State Park with a gazillion square miles of wilderness to get lost in. Further up north is one of my favorite out of the way state parks, Monahans Sand Hills.
A key scene in Fandango (burial site of DOM and location of the famous DOM rock which is still standing) is on the river road not far from Lahitas (sp??) and BBSP. If more of a dark drama is your taste in movies, check out The Three Burials of Melquiades Estrada with Tommy Lee Jones. It was filmed in BBSP and BBNP as well as Van Horn and other west TX locations.
West TX is one of my favorite places on earth, it's so bleak and desolate with miles and miles of nothing. Mysterious and vast, it defies most adjectives.
Enjoy your trip!