A reversed polarity on the garage outlet will energize the neutral side of the 120VAC circuits of the TT. Since the neutral is bonded(tied together) to the ground, the TT chassis will have 120VAC on it. By touching it, you become the path for current flow to the earth. A tester like RoyB suggests is a very worthwhile tool to have. It is also to check your house outlets to make sure nobody has incorrectly wires those receptacles.
Edit: I see you already checked the garage outlet.. Never mind.
I would look for and clean up any corrosion or loose terminals on any and all terminations. The terminals from the battery to the chassis or circuit breaker are good for getting lots of corrosion.