On later model RVs, the 12v system is running off the battery all the time, whether you are plugged in or not. The converter just charges the battery and kicks in more 12v power if/when needed.
In the older RVs, the 12v system is running off the converter only when plugged in. It doesn't get any help from the battery until the shore power is unplugged, at which time a transfer switch connects the battery into the circuit. If this transfer switch sticks in the shore power position, you will not have any 12v power when unplugged, even if the battery is full charged.
It sounds like this could be your problem.
Rich
'01 31' Rexall Vision, Generac 5.5k, 1000 watt Honda, PD 9245 conv, 300 watts Solar, 150 watt inv, 2 Cos 6v batts, ammeters, led voltmeters all over the place, KD/sat, 2 Oly Cat heaters w/ ox, and towing a 2012 Liberty, Lowe bass boat, or a Kawi Mule.