We had a lot of discussions already on this..
RPG (as in role-playing-game) could be used in any game, because you're actually playing a role of some character..However, RPG as gaming genre, has some criteria that
needs to be followed/fulfilled..
Main point is the system of leveling-up and building your character (statistics etc.)
Zelda, that actually has some similarities and elements of RPG, isn't actually an RPG game for the lack of that leveling-up system..Yes, you do find hearts, and you get upgraded weapons, but you don't build your character while battling (ie. simulating real life) so that leaves us with Action (you fight monsters, button mashing) Adventure (solving puzzles, using your head)..