Hunter 34 with an odd battery setup.
As you can see from my crude drawing a negative wire from the battery is going through the bulkhead and one battery's negative terminal is not even attached to anything.
I'm assuming that it was originally a two battery setup and someone added the third battery and never finished the install.
I agree with your assumption.
The battery with the unconnected negative is not even in the circuit. It's nothing but ballast.
I look forward to seeing how you clean this up and put that 3rd battery to work.