Did some more research and found this:
Though rugged actor Lee Marvin was a frequent guest on Johnny Carson's "The Tonight Show," it's unclear whether the exchange cited in this letter ever took place. What we do know is that the story can't be true, regardless of whether it was actually ever told. Here are the facts:
Keeshan turned 75 on June 27, 2002. The current version of the rumor above had been circulating long before that. This comment has been added by an anonymous forwarder
Bob Keeshan and Lee Marvin both served in the Marines, but never together.
Marvin was wounded at the battle of Saipan, two months prior to Iwo Jima. He received the Purple Heart, not the Navy Cross, for his injuries. There is no record of him serving at Iwo Jima.
Keeshan enlisted in the Marines in June, 1945 - four months after Iwo Jima. He never saw action during the war and never received the Navy Cross.
Entertainment legends are expected to tell legendary tales. It's possible that Marvin did relate this story, but its also just as likely that it's an urban legend that got attributed to Marvin to make it sound better. Not surprisingly, this chain picked up some steam in response to Keeshan's death on January 23, 2004, at age 76.
Urban legends that portray childrens' television stars in a situation or profession that is in stark contrast to their on-screen personas are common. Can you imagine dear Captain Kangaroo lying in the sand, clad in fatigues, dragging on a cigarette? Me neither, that's why legends like this endure - shock value.
Newer versions of the Marvin/Keeshan chain above now carry an interesting prologue that suggests another beloved Childrens' TV personality has a dark and deadly secret.
This one is completely the stuff of Urban Legend - The facts do not bear it out. (The same rumor has also been falsely applied to folk singer, John Denver." Rogers was never in the military - let alone a Navy Seal. And, he was born in 1928, so he would've been the oldest sniper in Viet Nam. It's no surprise that this bit of folklore resurfaced in the days following Fred Rogers' death in 2003 - and the Marvin/Keeshan chain seems a logical vehicle to tack it on to.