There should be a 120V ground (green) as part of the connection, and
then there should be a case ground that goes to the 12V neutral along
with the 12V positive and negative connections.
But, part of this has to do with whether the 120V ground is connected
to the 12V neutral. Nigel Caulder goes to great lengths to beat it
into anyone reading his book that these should be connected for
safety's sake, even though it can lead to galvanic corrosion.
The arguement is basically what happens if the 120V hot shorts
to the 12V neutral (or positive). Without a path back to ground
to trip cause an over current condition and trip the 120V breaker or
GFCI, the 12V neutral will be hot with 120V, just waiting for someone
to touch it.
Anyhow, this is a good book to get. I think the title is Boat Owners
Electrical and Mechanical Guide or something similar. The 3rd
edition is available from Amazon.
Mike