[prairieschooner]

I usually stay away from things that I have no experience with but.........When I am trying to find a freshwater leak I will use Food Coloring. Not sure that this helps but if you could find a way to use some Food Coloring on the exterior (again I am not sure how this would work unless say you used a bucket?) this may help isolate the problem.
Another thought..."the longer something is supposed to last...the harder it would be to get rid of"
good luck!