The negative or positive cable from inverter should not have a voltage if you have disconnected the shore power and stopped the generator before doing your wiring from the battery to the inverter.
It does sound like what you view as the negative, inverter to battery, cable may be connected to the positive terminal of the inverter.
1995 38' CC Magna #5280
C8.3L 300hp Cummins, 31,000lbs
Gillig Bus Chassis