1990 Class C Cobra Passport Ford E350 electrical ?

It's common for Class C RVs of that generation to have one chassis and one coach battery both located in the engine compartment. It's also normal that the converter only charges the house battery. If you buy a Trik L Start (easy Google) that will insure that your chassis battery gets properly charged when hooked up to shore power.

The basics. House system is 12 volts. One battery is for that. One battery is for the chassis. The converter charges the house battery and provides for the 12 volt house system. The converter is on a 120 volt breaker. The chassis battery is charged by the motor alternator. Don't know your system but the ALT MAY charge both. Convert won't charge chassis.

You say shore power does not get any of the house 12v items working, but you do say the converter does recharge the house battery, which does run the 12v things.

So where is the converter getting its 120v from if not shore power?

It appears the house battery is the second one in the engine room and it will be getting alternator power from the relay near the starting battery when the engine is on, but not when the engine is off.

The converter will not recharge the engine battery, only the house battery. You need a second (maintenance type) charger to maintain the engine battery if parked for a long time. That is normal.

You can replace the 1990 converter with a newer type. I tossed the charger portion of my Magnetek 6345 for a PowerMax deck model that fit in there ok. You keep the breakers and the DC fuse panel (but have to "jumper" the fuse panel so it is not "split" anymore as for the 6300). There are other choices too.

We went to the old Class C route from a 5er and a truck camper. (see photo in Profile). Watch out for weight on the rear tires! they build them overloaded to the rear, leaving nothing for your stuff. :( Can't be helped, but take it easy and you are ok.