Obviously I cannot know whether you had a leak, filling accident, or this really was simple condensation. I DO, however, know that you can get a huge amount of condensation under the right conditions: high humidity, really cold water in the tank.
I have no idea what sort of filling setup you have available, but if you can get both hot water and cold at the hose, you could adjust the mix temperature to be as close to ambient as possible. This would eliminate most of the problem.
But I, too, suspect you may have had a filling accident. I know my TT FW tank vent has a tendency to not discharge air as rapidly as I can add water. And unless I pay close attention to the filling process, I can create a real gusher while at the same time only having a half-tank of water. My tank is not inside any compartments, however, and all I get is water everywhere OUTSIDE my trailer.