It’s impossible to have a valid voltage reading of 110/220 and have that same electrical source not have a Hz (frequency) reading (assuming AC power). Your unit is 400Hz, so you need to make sure that your multi-meter (and you must use a good multi-meter) can measure frequency in that range. The convenience outlet, I assume will be 110v at 60Hz. But you should focus on the main lugs. Confirm you are using a good multi-meter and not depending on the on-board meters, which can fail.