Unfortunately, the Lee Marvin story is also fiction with a touch of fact. He was shot in the rear but not on Iwo Jima. He did not receive the Naval Cross. He did receive the Purple Heart. Mr. Marvin did tell the story about Bob Keeshan on the Tonight show or anywhere so far as can be determined.
So, it boils down to Lee Marvin saw action in the Marines; Bob Keeshan (also known as Clarabelle the Clown on the Howdy Doody Show) was a reservist in the Marines, was called up at the end of WWII but never saw action; Mr. Rogers was never in the military and never saw action.
http://www.snopes.com/military/marvin.asp (among other sites)