How to integrate a portable generator?

[Epicurean]

My solar charging system is quite simple and almost always gives me enough power. It involves solar panels, MPPT charge controller, and Li batteries. There's currently no provision for a DC or AC battery charger. But I'm thinking of doing an ocean crossing where besides the added drain of navigation equipment and AP, I may be in areas with heavy cloud cover for days at a time. So I'd like to get a little gas genset to charge the batteries just in case. Where/how do I plug that in to my system? In terms of gas consumption is the generator's DC output or driving a little battery charger more efficient? Thanks.