From lots of experience in this field, Karl, I'd be willing to bet it was the ground which had been lost, not power. Look for a broken wire inside the insulation or corroded ground connection.
Some people attempt to use the trailer hitch itself as the ground, and they're the ones who have intermittent problems. The rest of us have to troubleshoot their problems when we inherit the equipment.
-- You can either hold yourself up to the unrealistic standards of others, or ignore them and concentrate on being happy with yourself as you are. -- Jeph Jacques