It was 13.39 before I started the engine with all 8 batteries charging. Flipped off the inverter and started the engine and it dropped.
Have you looked at the Balmar regulator, it should have lights on it that will blink status and errors IIRC? Avanti will probably be able to fill you in on procedure to check it, and you can get a manual online if you didn't get one with the van. It should also be fused, so that might be blown.