I do believe that Uncle is Red Harlow. Dan Houser has said that the original Red Dead Revolver takes place in a different universe from RDR, but that some of the events of that story may have happened in the RDR universe. Hence all the campfire stories.
What makes sense to me is that the events of the original game are an embellished legend based on real events. RDR3 could tell the "true" story of Red Harlow culminating in him becoming the character we know as Uncle.
