I left the unit on the boat last Friday after returning from our anchor out. I left the banks tied together and on the portable 10A smart charger for a couple of days while I pondered this problem further. On Sunday, I returned to the boat and found the batteries at full charge. I disconnected the portable charger, turned off all loads and removed the ProMariner charger to bring home for bench testing.
I found a troubleshooting procedure on the ProMariner website and the unit passed on home 110V power. Now I started feeling better and felt I was closing in on the culprit. The ONLY components left to suspect were the 12V leads from the charger to the batteries. I had previously tested them for continuity and they all passed. I even disconnected them from the batteries and the voltage readings from the charger were normal.
I returned to the boat today committed to replace all 12V leads from the charger to the batteries. When I removed and disassembled the hot lead from house bank I immediately found the culprit.
Here's what I found when I peeled back the shrink tube:
Here's what it looked like when I removed the insulation...6 strands connected, 13 strands broken!
This weak connection passed continuity tests with the ohmmeter and carried voltage with no load when disconnected from the battery. But this could not carry and sizable current and resulted in my low current flow into the batteries and the failure to boost the bank voltage.
When I connected a new 6 guage cable, the voltage immediately rose to over 14V.
I believe I can label this Case Closed. Thanks to all for the helpful suggestions and support.