Airstream just loves to make things like this puzzling!
According to this PDF from Airstream
, the '74 Airstreams like BadKarma266's Tradewind should have a G248 lock and key, which is a double-sided Illinois key. Last Friday (before I found that PDF from Airstream) a locksmith told me that the Thetford door for my '75 Argosy's fresh-water fill had a "double sided Illinois lock with an H keyway." I'm guessing he just didn't examine it carefully and it's a G keyway, or maybe they're confusing and have G codes for H keyways. In any event, he didn't have any blanks for it, and I didn't yet have that G248 code.
In '75 Airstream supposedly changed to the CH 751 lock, which is single-sided, but your '74 Argosy has a single-sided key, and my '75 has a double-sided one. "Random" is a bit of an overstatement, but sometimes it feels like the right way to describe how they built 'em!
I'm going to try to find a G248 key this week... OutOfDoorMart is out of stock, maybe my local dealer can get one, or I can find a locksmith to make one now that I know the code.