Engine Battery Not Charging

So here's the latest and it's none of the above. While poking around today and checking voltage at various parts of the system, the coach batteries were reading 12.9 volts. The engine battery read 12.4. I checked the 9100 charger converter at the output connections and it did indeed read 12.9 volts. Not what I want for a charger in float mode. I have an extra Iota DLS 45 charger/converter that I have installed on a number of boats, including our own. This is an excellent smart charger and it can run the 12 volt system even if no batteries are connected. Once the Iota was connected and plugged in, the voltage on the coach batteries immediately went to 14.4 volts and the engine battery was reading 12.95 volts, much higher than I have ever seen it. So things have improved. I plan to check everything tomorrow and see just how much both batteries have charged overnight. We'll see. Chuck