Quite often the water door was a different key. The lock was supplied with the door and was used at random. I have not had a unit that had the water door be the same as the other compartments. Kevin, you may have had one that a PO got tired of having so many keys.
Your best bet is going to be to either take the trailer to a rally and have a key trying party. Someones may fit, or take it to an Airstream dealer that has been around a while. They always have a box of loose keys. If you know a GOOD locksmith then you could go that route, but I have heard of locksmiths butchering the lock, door and skin trying to get the thing open.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato