#12 for the branch circuits, 15 amp fuses. AS ran both a positive and negative to each device, and did not rely on the chassis shell for the negative line. (Argosy only used a + line and local negative to the shell). Limit how much you put on each branch 12 volt
circuit to about 10 to 12 amps max. I would run an individual circuit for the water pump, furnace, and refrigerator, even though none of them are large loads, it is nice to have them isolated with only one item on each of those circuits.
For the charge line from the vehicle to the battery at least a #10. Remember to fuse both ends of the charge line. For battery lines to the fuse panel, #6 or #4 even better.