A quick test. Remove the ground wire from the convertor and test the skin while the convertor is plugged in. If there is no voltage on the skin during that test I would suspect the convertor is bad and leaking to the skin through the ground wire.
If so remove the convertor and bench test it. Plug it in and measure from the convertor body to ground. There should not be measurable AC voltage there.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles