First, I'd suggest installing a 12v fuse panel with separate fuses and wire to your
furnace, water pump,
refrigerator (if 12v), inverter (if you build one in . . . I did, a 150w unit with a shore power/inverter crossover switch, and love it), 12v sockets, and
lights. It makes trouble shooting easier on down the road.
As for wire gauge, I'd go with 10g or even 8g from your battery to the fuse panel, same gauge as the mains for your inverter, then 12g to everything else.