"The King of Marvin Gardens" obviously referred to Jason (the Bruce Dern character), since he was the dreamer with the get-rich-quick mentality. In an original scene mentioned by some film critics, David (the Nicholson character), in his anecdotes about his brother in his midnight talk-shows, often called Jason The King of Marvin Gardens. That scene had apparently been cut later.
David was instead "The Philosopher King" - which was the original title of this movie.