DArtagnan makes a good point in it being hard to separate elements, there is no general rule, the higher percentage of elements needed to denote the genre go towards pigeon-hole description…but not necessarily or strictly so.

(a) - An adventure game could have 1 rpg element.

(b) - A rpg could have 1 adventure element.

…and of course a percentage of anywhere in between up to maximum. Going along with this theoretical make up you can see that somewhere in the middle a game could be either or one or the other.

(c) - A 50% adventure and 50% action game could be an action or adventure game.

(d) - Now introduce one rpg element or small percentage thereof to any ratio of the above and instantly, at a stroke, you have a rpg….like it or not! - and that is the way many "rpg's" have been touted in the past. Adding an rpg tag to a game has been proven over the years to increase sales. One prolific example being Diablo-1, which was without a shadow of a doubt an 'rpg'.