I seem to recall an episode where Chachi was destined to sing along with the Beach Boys, but had diabetes. He also tried to hide it from Joanie, right?

that was in episode #250- Good News, Bad News (7/19/84)

"Chachi decides to give up an opportunity to tour with the Beach Boys after he receives startling medical news."

i think that it was more that Chachi was scared to tell Joanie... but it was kinda hard to tell what the heck was going on in his head...

I think it was Fonzie he kind of tried to avoid telling. I'll have to check it out someday. He did end up going on tour with the Beach Boys, and everything turned out just fine.

I thought it was Joanie. But I think you are right with the latter part. It could be Fonzie. But always thought Joanie for some reason.