First of all, if you installed the converter, go back and check all connections to make sure they are clean and tight. A bad ground can lower current reaching the battery(s).
The voltage you read at the battery will start with the actual voltage of the battery when charging is started. If the starting voltage was only around 10.5 volts your battery was close to fully discharged.
Many converter/chargers have trouble starting charging on a very weak battery. A portable charger set to "2 amps" is needed to slowing get voltage of weak battery to around 11 volts. Then use the converter/charger.
It seems after a few tries your converter/charger did get started charging your low battery(s). The voltage reading should keep rising from that low reading to 13.2 volts and eventually either 13.6 volts (or 14.2+ volts if multi-stage charger). Charging should continue for 12 to 24 hours on very low batteries.
Your earlier replies do not state you charged this long yet. If the reading at the battery-terminals never went beyond 13.2 volts the charging portion of the new converter/charger is not functioning. The converter section will output 13.2 volts @ battery-terminals without any real charging taking place.
