You may be suprised to find that the water fill is not all that easy to get to to pull the lock.
Time to have a contest!
First forum member that has a key that fits (there are only so many combos) gets ??????
Or take it to a vintage rally and ask the attendees if they would come over and try their keys. Getting a copy made is cheap.
You may be asking why is the water key different...........
It was for saftey sake, as it was explained to me by an elder member. The exterior compartment keys on many years were ALL alike. My keys opened many other owners trailers, but the water doors were from a diffrent vendor, and this meant your key would only fit 1 in ?? trailers. This minimizes the risk of someone contaminating your water supply.