I've had a NextGen 3.5KW generator for several years. It does not have any 12V output.
This is not a problem. The reason for running the generator is usually to recharge the batteries
(and to provide 115VAC for other purposes). Thus whenever it is running, so is the installed battery charger(s).
I have two battery chargers: a high-end Victron MultiPlus (120A capacity) for the house battery bank and an Iota 55A which is dedicated to the windlass
battery bank up forward.
I keep the starting battery charged by means of a little device called an EchoCharge. It works flawlessly. It senses a charging voltage on the house batteries
and bleeds off some current
(up to 15A) to keep the starting battery charged. A Balmar
DuoCharge does the same thing, but is more sophisticated and can handle up to 30A (neither of which is needed in my case).
Suggest you get an EchoCharge or a DuoCharge to keep your starting battery up, and use the Iota for house bank charging if you don't already have a good charger for the house bank.