Hard to tell on a 84 Class C chassis but typically that kind of setup will do the following:
Engine on: Alternator, starting battery, and house battery at same voltage, your 14.4 sounds right for an alternator source
Engine Off and using generator/shore power: House battery at converter voltage (13.8 in your case) and engine battery resting (call it 12.6). Typically the converter does not charge the engine battery. You might have a boost switch to help the starting battery by manually connecting your house and starting battery together if the starting battery is low.
Sounds like your isolator has failed is not working (i.e not connecting house to alternator when voltage > than house battery voltage)
Also getting a reading of 12.28 on the house battery at rest is very low (approaching 50%). You need to fully charge that as soon as possible. With a converter operating at 13.8 that couuld take a couple contineous full days. You need to plug in for a while.
