Just make sure you are on the lowest ohms scale, especially if you are using an analog meter. On the higher ranges, your meter may measure full swing through internal circuits such as a light bulb. Also make certain the shorepower is unplugged first. Ohm meters don't like voltage/current.
Another test is to insulate the red and white cable ends, then plug the unit in and check polarity from your converter, but I would de power any other 12 volt appliances, like radios. Our 12 v batteries make better regulators than some converters.
2016 Ford F-350 Lariat CC, 4x4 SRW. B&W turnover with extender. 2015 CC Silverback 29RE, fitted with Reese Goose Box. Amateur Extra Class K5TS, 2016 nights camped 16