Chuck Matthews location on the map
Chuck is at a house northeast of camp, just a bit more north of Emerald Ranch. He’s already saddling up behind the house in a shed when you get there, and he’ll immediately run away on horseback.
Follow him, and ready your lasso. Remember to hold the L2 button to keep him tied up with your lasso. Just search him once you have him on the ground, and you’ll find a treasure map to all of his worldly possessions.