Hello, Yes your husband is correct, Yamaha generators come with a charge cable that fits the angled plug in the frt of the generator, this is used for recharging batteries. The Plug and charge cable are designed specifically for this purpose.
it is extremely usefull and alot of Generator customers forget its there or are simply not aware of its intended use.
Charge Cord is $44.51 in stock
Shipping $18.00 anywhere in N.America

Owning a Boat and RV I am sure you will get good use of this feature.
Regards