You did not say how long you run the generator. It will take many hours to recharge a battery from the converter. Overnight if it is really low. The engine alternator puts out much more. If you really want to charge from the generator the fastest way is to carry a good battery charger and with the generator running hook the charger to the battery, even this will take several hours if the battery is very low. The fastest way is the engine. Things like a coffee maker that produce heat use a lot of battery power. We always use the stove when boondocking.
Good luck,
Wayne