Well, we wish we had known this group of experts before we went through the battery incident.
The boat has 4 - 12Volt 90 Amp Deep Cycle Sealed AGM batteries. Two are for the house; 1 is for the engine and the other one works for accessories like starting the generator; running the thrusters; operating the windlass anchor; and the raw water pump. This is the one that was completely dead.
We thought it was last to get charged and the last one that mattered. I'm not sure the intricacies as to what is in parallel or what is in-line.
The boat has a 150 HP Cummins-Mercruiser straight inboard diesel with about 850 hours. It is essentially a 35 foot boat stuffed into 25 feet. We have all of the features of a bigger boat minus the space. Quick list in no particular order: 75 gallon fuel tank; 30 gallon water tank; radar; bow and stern thrusters; windlass anchor; autopilot; queen berth forward; head with shower; propane fired stove top; air conditioner; heater; trim tabs; battery charger; chart plotter. It's a "boater-home".
We have both taken the boating safety course. We would really like to hire a captain to show us the ropes in the form of anchoring and running the generator and other things. We are learning more each day.
Anyway, thanks again for the well wishes. Looking forward to learning from this forum.