I've traced the source of that
Robert Mitchum/Jim Morrison conflation to an article in
THE ATLANTIC (the
article itself ripped off throughout the lawless plains of the "internet") that seems to quote the Lee Server biography of Mitchum in part, though I'm not sure there's a footnote or credit given, but there probably is, somewhere, and if there's not, who cares,
life is just a big ball of chaos.