It didn't flow thru the insulation of the cord. Did you have the cord plugged into an extension cord? This would allow water to touch the prongs.
However, it sounds like your trailer frame was "hot" for some reason. If the trailer were properly grounded it should have tripped the breaker but it didn't. I would make sure the ground connection in the pedestal, cord, and RV breaker box is secure. There should be a bare or green bond wire connected to the frame of the trailer.
Also, make sure the polarity of the outlet you were plugged into is correct.