Uh, Neutral is a 120vac side wire
Positive is a 12VDC side wire
The two should not be related so a "no reading" is normal though on shore power with the inverter connected things might be different,, You may also find a 60 volt difference between a 12 volt side and neutral on generator, But that is a false reading.
Converters come in several flavors, one is UNFLITERED and those can give a false voltage reading when not under load.
However that said, What is the make and model of your converter. Some of them needed upgrading the day they were put in the box at the factory.