Seeing as Manhunts Carser City has been mentioned in the GTA universe, I dont see why RDRs map wont be. I doubt we will ever see parts of it in GTA V but we could definitely get references or maybe characters from that area. As ZCAB said, new austin is based around texas, arizona and new mexico. Id say west elizabeth was more of a stretch between montana and winconsin. Im guessing blackwater is south of chicago, where his father went blind during a bar fight, and tall trees be the mountains in montana.
I do doubt the possibility of seeing any relation to RDR, though.