You should be able to find gas every 100 miles or so until Del Rio.
Then Langtry, Sanderson, Marathon, Alpine, and Marfa.
Don't leave Presidio without gas.
When going to Big Bend gas is available in Study Butte, Panther Junction,and Rio Grande Village.
Keep top half of tank full and you should be alright.
Richard Gibbens has some really good Ride Guides vor the Big Bend and Hill country areas.
They are available here http://texasadventure.net/cool-stuff/ride-guides
ADV Relay Rider 08 (The Texas Connection)
Ride Pooie Ride
Don't stop riding until you get to the crash , you might save it!