Some portable smart chargers need the batts to be over 4 or 6v or they won't run at all. First you need a dumb charge to get the batts up a little, then the smart charger can take over.
Can you run booster/jumper cables from the converter battery lugs to the house batteries direct? Can you do that from the engine battery?
Park a car next to the house batteries and jumper from the car?
No idea how to fix it so it won't happen again though. MHs are too complicated for me!
Edit--Oops , I see you are in business--never mind. :)