Might not be a problem at all. Your converter is probably a 3 stage WFCO unit. If the batteries are already charged i.e. after driving home, the converter will only be outputting 13.2 vdc as a "maintenance" or "float" charge. When you're plugged into shore power your meter will read the shore power voltage from the converter. When unplugged from shore power the meter reads the actual charge voltage of the battery.
The converter will up the charge voltage depending on the battery state of charge from 13.2 to 14.4 vdc.
https://wfcoelectronics.com/product/wf-8955-55-amp