Current practice with Airstream and for the most part the rest of the RV industry is to run dedicated circuits for any high-amperage appliances that are present (water heater, air conditioning, microwave), and put everything else on two circuits, divided based on whether the outlet in question has to be GFCI protected or not.
You can use 12-2 with ground for all the circuits and breaker them at 20 amps, except that the water heater is supposed to be breakered at 15 amps.
On the 12 volt
side of things I would use 16 gauge wire for most circuits and plan the circuits for an 8 amp load and breaker everything at 15 amps. Larger loads should have their own circuit with larger wire and a larger breaker. These will include any cigarette lighter type power outlets, the tongue jack, water pump, breakaway switch, and furnace.
Unless you have an inverter you can use 6 gauge for the connection to converter and the connection to the batteries. I use 8 gauge for the charge line.