Does anyone know if Charles' last name was ever mentioned on the show? It doesn't appear in any of the cast lists that I've looked up.

Nope. He never had a last name. It's one of the greater TV mysteries. http://www.sitcomsonline.com/ubb/wink.gif

Did anyone ever check up on what someone posted a while back about Charles' name being mentioned on one episode?

I believe they said that his last name was mentioned on the one where Donny Most of Happy Days guest stars and wins money off a lottery ticket that Mr. Powell had bought. He then gets run over by a truck at the end of the episode.

Is Charles' name mentioned or not? I saw that show and did not notice anything.

I don't believe so, Charybdis. During the series finale, the cast pokes fun at the fact that Charles never had a last name, so I'm certain he never had one. Charles was Charles and Lilian was either Lilian, Mom, or Aunt Lillian. http://www.sitcomsonline.com/ubb/wink.gif