One more thing. If you have never been to the park, it is a MUST see. From San Marcos, it will be a very long drive. You may want to consider camping at the park overnight to get the full experience. Lodging is non-existent in the region. The closest would be Del Rio, Rocksprings, or Sonora. There is limited, primitive camping at the park and it is not for the faint of heart. I recommend going in October/November or in the spring. The park is 25 rough miles off of highway 277. Take dependable transportation. And, no, your cell phone will not work at the park. You can get fuel at Loma Alta though I would tank up at one of the cities mentioned above just in case the ice house in LA is closed.
I'm publishing a novel titled, "Vinegarone," in September. Check my website for stores where you may find it. I would be interested in reading your research when you are done.
Good luck. www.dbcarlyle.com