My WF-8900 Power Distribution Center for my 120VAC and 12VDC circuits that houses my converter/charger is wired like this diagram of the WF-8700 series unit..

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You can notice all of the GROUNDS for the Trailer 120VAC receptacles are wired to a common GROUND BUS BAR which has one connection going to the Trailer frame. This is all presented to the shore power GROUND connection.
The NEUTRAL leads also go to a different common bus bar which is isolated from trailer frame ground and goes to the 120VAC source NEUTRAL connection of the shore power cable.
The HOT wire from the shore power connection goes to the Circuit breakers to power up the appliances and 120VAC receptacles...
The NEUTRAL should NOT be wired to the Negative Battery terminal which is a Trailer Frame Ground connection. I suspect someone has added this connection for some reason...
By code the NEUTRAL and GROUND connections are suppose to done at the 120VAC SOURCE connections.
Roy Ken