Perhaps a short on his trickle charge wire resulted in overheated wire which melted the insulation and thus caused arching / sparking with other wires it came in contact with which resulted in the fire. The overheating of the trickle charge wire could also have resulted in the lithium battery over-heating and resulted in the fire. These trickle charge wires can get extremely hot when there is a short.
I have had personal experience with over-heating charge wires.
To explain why the bow was also on fire, I feel this has a lot to do with the ventilation tunnels of the boat from stern to bow. Also, later model boats are accessorized with ventilation fans in the compartments which could have contributed to the spread of the fire from the stern to the bow???? Perhaps the fans were left on???
Now, with the respect to some of the tackle missing, I do not recall this being stated in any report submitted by the investigators?