I kindly disagree with some of the posters above. I think that there isn't such a thing as a hard "campbell model" that has to be applied this way and not that way, much like I think there isn't a clear cut frontier between mythology and other matters of our lives.
The hero's journey as described by Campbell is an attempt to explain what most heroes (and thus every single human being) go through in an archetypal manner. Thus, many examples can fit this explaination. For instance, D&D heroes going down into "the dungeon" are leaving mundane life to go through different trials (encounters), end up confronted to the climax of their quest to take back whatever things they found (gold, knowledge etc) back to the world above. D&D in that regard fits the hero's journey.
Most of the RPGs fit this concept of journey through their very nature, particularly narrative. Roleplaying often takes the form of various types of conversations and obstacles involving PCs and DMs. These are steps that go from the hook to the climax of the adventure. Therefore, RPGs are "heroic", as per Campbell's definition, by their very nature.
The Hero With A Thousand Faces illustrates what I am talking about. Luke Skywalker goes through a hero's journey, by Campbell's own words. So do most characters of fantasy roleplaying games, weither we're talking about D&D, Vampire, Star Wars, Warhammer or Conan. Doesn't matter. What matters is the evolution an RPG character goes through, from an initial "normal" life to an awakening to something challenging the character's ideas, and the realization of what his/her life/adventure teaches him/her in the end.
And it doesn't have to be expressed in a pure "roleplaying", as per "storyteller roleplaying" way. Gold for instance represents a mythological purity, particularly the purity of the soul, much like virgins. This is why the Knight, for instance, kills the Dragon (desires and passions unbound, raging within one's mind) to rescue the Virgin and/or take its treasure (find the peace of mind, the awakening of the soul).
outside literature/mythology. As has been discussed many times, games are not literature and vise versa. Literature is by definition created by one (or in some cases two or more) writer, with a definite beginning, middle, and end in mind. Games must have in their design room for the players to make decisions and change the outcome of the adventure.
Mythology is not literature. Mythologies are formed with stories told again and again through generations, impregnated with what cultures and human beings keeping these stories alive hold for true about human nature. Ultimately, just like RPGs on a microscopic scale, mythology is not the fruit of a single individual's imagination and aspirations, but the combination of many voices through the ages.