BamamomRTR, sorry but I had to run off for a few hours to assist with a neighborhood fish fry.
I see there have been several "cooks in the kitchen
" trying to help with your problem since I last checked in. I hope you aren't completely confused and discouraged at this point.
My conclusion (guess?) is that your converter is completely dead and has helped drain your batteries as well.
Does your DH have a standalone battery charger by any chance?
If you could place a battery charger on the pair of 6V batteries it could function in place of the converter. You would need to place the charger across the two batteries as if they were one large 12V battery. You also need a live outlet to plug it into.
If the batteries are completely drained, it may be necessary to actually disconnect the short cable between the two six volt batteries to prevent them from overloading the charger.
BTW - That Radio switch is there to power the radio from the house batteries when parked and ON shore power without risking the depletion of the chassis battery. The other position powers the radio from the chassis battery via the ignition switch.