Fifth wheel 12 volt trouble shooting

IS your 5vr 50A?

Shore power and gen are separate sources (feeds) of AC power UNTIL transfer switch then they are same out of transfer switch

You have lost one of the HOT legs on shore power going to or in transfer switch
Shore power only powering one bus in MAIN AC Panel and converter feed is NOT on that bus....hence no 12V DC

Gen feeds both Hot legs thru it's side of transfer switch so your MAIN AC panel buses are hot and feed converter....hence 12V DC

Your battery is shot OR disconnect switch is OFF hence no 12V DC from battery

If the batteries are good and have good connections, you should have some 12 V power ALL of the time. Even if the batteries are mostly discharged, you should have SOME voltage; they seldom go completely dead.

So.....if you have NO 12 V power when connected to shore power, then the batteries are bad AND the converter is shot too.

You NEED a meter......or a shop visit.

Doubtful of being converter as it works with genset.
To clarify genset is an on board hooked up through transfer switch? Transfer switch could be the problem.

Bad battery, pretty simple to check with a volt meter.

Did you have 110 on shore power?? Do you have a transfer switch for shore power & generator? Your 12 volt comes from 2 sources battery power and converter power. Shore power and generator, power the converter. So if your getting 12 volt on generator power and not on shore power the power source from shore power to the converter is at fault.

Time to get out the voltmeter. You have one, right?