In that case with load tested batteries and they are boiling you don't want the Magnum to be charging them which is why I suggested a portable charger. There are some decent 3 stage auto charges of say 20A+ for reasonable prices.
If you disconnect the DC cable to the Magnum ONLY then you'll have house power and you have the alternator and portable charger to charge them. The AC will continue to work when on shore power.
If the Magnum powers important AC circuits then they will need to be bypassed when the Magnum is removed.
Given what you've posted the best approach is likely to replace the unit, repair and then sell the old unit.