Disconnect the house batteries entirely. The Magnum charger should make your 12v just fine. If still having problems, it will be possible to trouble shoot that.
A bad battery might create problems for the charger in the inverter/charger. Your symptoms suggest another problem though.
You only need house batteries if you need to survive periods with loss of shore power and go onto inverter for 120v and battery for 12v (including the inverter)
You could be having shore power problems or transfer switch problems. Disconnect house batteries to remove them as a "distraction" for going trouble shooting, since they don't even need to be there right now.
