David,
Your scenario describes so many possible failures that nobody reading will be able to do more than maybe tell you where to start.
The fact that you loose 12V system when not AC powered means that the house bank has a problem like a bad connection.
The house battery bank should never be 18 volts.
The most converter fans do not run full time, only as required.
The fast voltage loss when loaded by the inverter says that the pair(?) of GC2s are not charged to full density.
You changed out the house bank? Are you sure you got it all put back together correctly?
If you can start the APU and run the jacks, then the house bank can't be that dead.
Can you get a real schematic from Forest River? Do you have a buddy that is juice smart? Offer to buy him dinner if he brings his meter.
Matt
