One of our readers, Dick Smeeding, had one of those things that could have sidelined his trip, but did a work-around. When Dick's power control center suffered a malfunction, the result was that his "house" batteries wouldn't charge. The starting battery system was fine, so he could continue to operate the motorhome engine system, but as far as accessories in the coach, he couldn't charge them when plugged into shore power, nor from his onboard generator. Yes, he could still charge them by firing up the motorhome engine, but he found that to be too loud and fuel wasting.
File his solution away for future reference – you might need it yourself. Dick made his way to a nearby auto parts house and picked up a relatively low current battery charger – a 2 to 6 amp critter. He also bought a DC accessory power plug – you know, the kind of plug that fits into what us old guys call a car cigarette lighter outlet. Between the charger and the plug, he spent less than $50.
Back at the rig, Dick pulled back the insulation from the wires coming out of the power plug (this one came prewired), and observing polarity, attached the wires from the plug to the alligator clamps on the battery charger. He also slipped a toilet paper cardboard tube over the positive clamp to prevent electrical shorting. Now with the onboard generator fired up, he plugged the 12-volt end of the lash up into a 12-volt power outlet, the battery charger into a shore-power outlet and, hey, presto! Dick has power flowing back into the shore power batteries. Mind you, this is not a huge push into those batteries, but it is enough to keep things going.
Why not hook the charger directly to the battery bank? It's not always easily accessed, and foul weather may make it unappealing.