I think the RO was out of line in the way he treated you, but not out of line in making sure you understand the rules. Yes, he was assinine about it, but he did his job. I have been downrange and have looked back to see people monkeying with guns. I have a friend who was downrange when a rifle was fired. Events like that can make you appreciate the RO who is focusing on his job instead of jaw jacking with his buddies.
FWIW, are cars always parked that close to the line? Every range I have ever been to has had a buffer zone of at least 30 yards between the line and parked cars. That in itself can create this problem.