Nexus class C battery issue

I would venture to guess that plugging in your motorhome and allowing the converter to charge the house batteries will only give you a minimum charge, perhaps just sufficient to start the generator, and if the batteries are too dead, or not salvageable, repeating the same process will only result in the same results you are getting. If your batteries are more than three years old, consider changing to 6 volt Trojan batteries, or at least, charging your batteries over 24 hours with a real charger, not a trickle charger or converter.