"San Antone" was written by Barry De Vorzon and sung by Denny Brooks. De Vorzon has written lots of pop and country songs as well as soundtracks. He even wrote the soundtrack for one of the greatest movies of all time: Night of the Creeps. Brooks was one of John Denver's backup musicians and recorded some stuff on his own but never achieved much fame.
This song was part of the soundtrack to Rolling Thunder (1977), which was partly filmed in San Antonio and which I've never seen, and then again was used in the opening scene of The Ninth Configuration (1980), which I have seen and is why I know about it.
This video includes the full song, only part of which is used in The Ninth Configuration, and some edited scenes from the movie.