Here is how I solved the problem. I have only room for a single 12v battery in the battery compartment and I didn't want to have batteries in other locations.
First, I felt I had to know what the battery is doing so I installed a voltmeter and ammeter to monitor the 12v system.
The OEM converter was useless so I installed a PD 60 amp converter and put in bigger wire, #4, from the converter to the battery.
One battery is still a problem but at least I know the state of charge and can deal with it before it becomes a problem.