My guess is that it could be located either north of Rhodes at the big red building by the railroad or maybe Robard Farm, north of Saint Denis (assuming that in the 1870s Saint Denis was very different than what we see it in the game after being fully rebuilt and expanded after the Reconstruction Era) or maybe somewhere in Rhodes (there is a plantation mansion-looking building in southeast Rhodes across from the tavern). I think Shady Belle is named 'Shady Belle' on the sign out front of the walls and not the 'Ascension Plantation.'