I've checked some of the "teenage runaway" movies from that era, and it's not
Born Innocent (1974),
Diary of a Teenage Hitchhiker (1979), or
Dawn: Portrait of a Teenage Runaway (1976). This one seems to be a play off of those, but one where the hitchhiker ends up being the manipulative perpetrator rather than the defenseless victim living in a harsh world. A nice going against the grain of established cliche that a lot of those "message" movies from that era were guilty of, which is part of the reason why I really want to see this one again.