Re: Isolated battery's forward for specific tasks...
As hellosailor said, 12 awg is fine for fans.
But - do you have any 12 volt interior lights forward? If so install a small terminal block at the end of the light feed and run wires from there to the fans.
As far as a windlass battery - moving a battery to charge makes no sense. And for a windlass the charging wires to a forward battery are almost as heavy a gauge as the wires run forward if using the main bank. Best to run the proper 2/0 or whatever wire and have one less battery to worry about.
Living aboard in Victoria Harbour