I'll go against the "right hand wires" theory in this. It appears the neutral bus was connected to the load center correctly with the three mounting screws. It appears that every wire has lost jacket material from heat, the right side being the worst. I would guess that the cause is a loose connection within the load center to the shore cord or a failing shore cord that allowed connection but a lot of resistance.
I'd suggest to check resistance of the whole neutral path from the male neutral plug end to the neutral bus.
