If you run the house batteries down, and don't have a source of 120 volts ac for shore power, then start your motor home engine. After a few moments for the engine alternator to replenish the charge in the starter batteries, depress & hold the "auxiliary" switch (often a picture of a battery) for several minutes at least. This switch is one of many on your dashboard. It will allow the engine alternator to charge the house battery. After a few minutes, (patience please), while holding the "auxiliary" depressed, also go thru the procedure to start your house genset. It should turn over & fire up.
Once the genset is running, release the "auxiliary" switch - it's job is done & the genset will complete the house battery recharge.
This works the otherway as well. If you discharge the engine batteries, get your genset running & use the "auxiliary" switch to renew the engine battery charge.
"They" warn not to overdo the practice however, because you are paralleling different kinds of batteries which could overstress one or the other. But for emergency recharges or starts it's OK - the switch is included for a reason!
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