RPG doesn't really fit this game to be honest. It'd okay as unlockable weapon that you can get by clearing the game but as a regular campaign weapon? Nah.
I object. It would really kill the vibe to be useing a RPG through the campaign even if beaten already. Although it could work as a one-time use in a slow-mo style moment cod style where you have to use a RPG to destroy a incoming chopper or truck. One shot only just for that part.