No 12 Volt without converter

So my recently acquired 2002 Jayco Qwest 253D fiver suddenly has absolutely no power when not connected to shore power.  Both batteries are showing 12.5 volts, so seem good, but not getting charged by converter.  All electric works fine with shore power, but no 12 volt at all without shore power.  All the regular 12 volt fuses are good.  There is no battery shut off switch as i haven’t gotten around to installing one yet.  Someone told me there is an automatic disconnect switch that might have gone bad, but i can’t locate one.  I’d appreciate any help anyone can provide.

Thanks!