After the War, many young men left the Old South. Some went to Texas, some farther West. Doc Holiday was one of those. Military experience, but had enough of war. Still could act in a deadly manner if required. Shane may have been one of those. If so, possibly from Georgia.
Zane Gray also wrote a biographical novel of George Washington as a young man, and put a lot of the old frontier ethics into George, even though the frontier was a lot closer to the east coast then.
Doc Holliday was the fiance of one of Margaret ("Gone With The WInd") Mitchell's cousins. I believe she based the character of Melanie on this cousin. He left her to go West and she never recovered. I read something about this in a biography of Margaret Mitchell.