I taught this to a class a few years ago and I would class it as realistic historical fiction. When labeling a book into a certain genre, do not be afraid to put it in all that it fits--just make sure you can explain why you put it where you did,
Characters like Onion John lived and were part of the Antebellum south. Even though there are some elemenets of the story that are true, it is not anyone's true story, so the fiction elements reign supreme. I would first call it historical fiction with a secondary label of realistic fiction.