This was posted by Betty Johnson on alt.humor.puns. I have not been able to determine if it is one of the original Feghoots.

I believe one episode had Feghoot travel into the future with a couple of vampires, who discovered to their dismay that the only life form existing on the planet was a race of giant turnips. Feghoot survived, but his traveling companions both died.

As everyone knows, you can’t . . .