Eric it may be that the pump out hose was brought down into the boat. I used to work a pump out boat here in the north east, and sometimes that is what had to be done. We would go directly into the tank or sometimes the owners had screw out plugs where we would attach the p/o hose to!!!!!
Honestly, this does not sound appealing and I can find no valve or pump out that would allow this. I wasn't able to do much at the boat this weekend but I was able to confirm that there is DC current in the wire running from the box. Next weekend I'm going to tear the whole apparatus apart and get to the bottom of this mystery.