i consider a rpg game to have at least a leveling system whereby gaining xp will earn you better skills and attributes etc. Essentially you build a stronger and more 'personalized' character throughout the game.
Other aspects of an rpg include:
- customization (such as physical appearance, personality, etc)
- lengthy single player quest
- dialogue (lots of conversation)
- choices that lead to different paths or outcomes
As I said before, xp and some kind of leveling system is a critical aspect of a rpg game though games can have a leveling system and may not be considered a rpg game (e.g., bioshock, dead space, etc)
haven't played RDR so I can't comment explicitly but I would typically call than an open-world game like GTA or infamous, and not a RPG.
