Tiago and Stephen, this poses an interesting approach to how one thinks about and incorporates the idea or concept of the “monomyth’, or as is most often thought about with the term: “The hero’s Journey” in a number of ways. The reason is, as you have surmised, you have to go back to where Joseph deduced or formulated this concept as a personal approach, not just societal; and I would suggest 2 sources if you haven’t already pursued them. One at the top of the list would be his “evolving” idea of Jungian themes; (especially that of individuation); and the other would be that of: “Heinrich Zimmer”; whose ideas affected him profoundly. (After Zimmer died in 1943 Joseph spent 12 years transcribing and editing Zimmer’s work for publication to keep it from being lost. Joseph mentions in Michael Tom’s: “An Open Life”; on pages: 122-123; how he felt about them both; and that he was not a Jungian, but a “comparatist” who was more interested in cultural diffusion than Jung was but used him as a source for interpreting symbols.) He clarifies who he considered as his “guru” as Zimmer because Zimmer gave him the courage to interpret out of his own understanding of their commonality. But he saw Jung as his guide to let the myths and symbols speak or talk to him from what he had already gained from his own background of research by saying although you run a risk when doing so it is the risk of your own adventure instead of gluing yourself to what some else has found.
One of the things that might be helpful when cross-referencing some of these terms like what the “hero” and the journey/adventure represent in a Jungian sense is the glossary link I am leaving: Daryl Sharps: “Jungian Lexicon”. This also includes definitions of “individuation”; (as a process); and other terms which might help give a better sense of Joseph’s thinking before he wrote: “The Hero with a Thousand Faces” in 1949.
Also, the Foundation’s: David Kudler may be able to help add further background if he is able to stop by from his editorial duties. Stephen knows more about these things than anyone I know of, but from what little I know Joseph had not actually come up with the idea until his conversation luncheon with his friend: “Henry Morton Robinson”; and a book editor about writing a book on mythology. Campbell replied in the book and film: (The Hero’s Adventure): “I wouldn’t touch it with a 10-foot poll! The editor then asked: “What would you like to write about?”; to which Campbell replied: “How about a book on how to read a myth!” (Thus: “The Hero with a Thousand Faces” was born and came out of his lectures to his students in 1949. By 1971 Campbell was already writing things like the preface to the “Portable Jung” and many other notable scholarly works on a number of other related topics; (but as a “mythologist”; not a Jungian scholar or analyst); and a number of Zimmer’s works came out of this earlier editing of Campbell’s research such as “The King and the Corpse”; “The Philosophies of India”, “Myths and Symbols in Indian Art and Civilization”; just to name 3 right off the top as an example.
In the larger context of Campbell’s thinking my suspicion is Joseph was looking for a common symbolic motif that kept re-appearing in myth after myth that crossed cultural boundaries but was “psychological” in nature, and in Jungian terminology could be recognized as “The Self”; not to be confused with the ego; but as the central archetype or regulating center of the entire psyche which is seeking to know itself throughout the evolving life stages from birth to death. In other words: “The Hero” as individual; whether male or female, not to be confused with the “archetypal images” through which this process is engaged or summoned forth; (aka, the call to adventure), in whatever life crisis or zeal of the heart presents itself or is made known. In other words, “you as yourself” are the Hero; not the forms in which it reveals in you symbolically; but the essence of you to know and express that which is within you as your highest and unique potential.
If you are already familiar with some of these concepts, then I hope my humble and rather clumsy attempt is not too confusing; but this is my understanding of Joseph’s Hero Journey motif. We all have our individual “Grail adventures throughout our individual Journeys”; and your quest to find “this particular origin” is a noble one indeed. I hope this is helpful in some small way. Best of luck to you; Sir Knight! Namaste