My inverter install includes a set of high amperage disconnect switches, one for each battery bank. The OEM battery disconnect is just for the relatively low loads for "house" requirements.
I can't understand why it is necessary to not keep the OEM 12 volt system in place, unless the battery bank is going to be 24, 36, or 48 volts.
If bank voltage is not 12, it would be a simple matter to include a small AGM jar to power those relatively tiny loads any the OEM converter may be left in place to charge that AGM.
Another solution would be a dc to dc converter.