BFL, if you have only done it the right way from the beginning, the way that Mex suggested - with controller to Pos bus in a dry and clean location inside the trailer, with a Pos cable of trailer system to the same bus, so that a Pos cable from Pos bus would be the ONLY cable to the battery, with a proper lug on it - you wouldn't have made this discovery and we wouldn't have enjoyed this amusing read ;)
Not only because with only one lug on battery terminal there would've been less chances of corrosion, but also because a poor contact on that only cable would've been immediately noted in your loads behavior.
Getting an AGM would've also helped to prevent the corrosion. Not that I suggest getting an AGM when you can do without them.
