Well, this assumes that Campbell is accurate! Campbell is playing fast, loose, and vague with the evidence, but that's a different story.
The ancients as far as I am aware did not recognize Campbell's "monomyth." They did, however, recognize a few things. First, at least with Mediterranean and Near Eastern cultures, they all engaged in a process of _interpretatio_ , through which various heroes and deities were seen as the same. Heracles was Melqart, though the Greeks called both "Heracles", one "Tyrian" and the other "Greek." (Herodotus, 2.43–45, Cicero, _De Natura Deorum_ , 3.43)This was very common.4
Another process common especially during the Hellenistic era was something called "Euhemerism," named after Euhemerus, a Greek philosopher who thought e.g. the gods were once kings and mighty men who subsequently were worshiped as gods. This presumes an interpretation of similarities, though not of journeys, but of kingship.5
Does this answer your question?