The other mistake I see on some older boat is 2x 30A inlets, but only 1 neutral bus bar in the panel instead of a separate neutral bus for each inlet. That will also cause GFCI trips, but it's fairly easy to correct, the neutrals just have to be split and each circuit moved to the correct bus bar.
The other common neutral-grounding error occurs on boats where an inverter was added and some of the circuits are served by the inverter. Often, whoever installs the inverter separates the AC hot wires, but doesn’t bother to put the neutrals on a separate bus bar tied back exclusively to the inverter. Even the LED jewel lights adjacent to each breaker need to have their neutrals on that bus bar.