When I was rewiring my boat, I made a wire gauge calculating spreadsheet to help with determining exactly how much of my paycheck was going to go into copper. I adapted it for wiring my Sprinter conversion, and figured maybe it might be of interest here. I'm an EE, so you can trust me.
(this is a lie, don't trust EEs, trust EEs who specialize in vehicle wiring -- which I am not. Use at your own risk.)
You give it the run length, whether it's one-wire or duplex, the maximum current, and whether it's critical or not. Critical means "I need this equipment to operate the vehicle safely" and is a bit subjective. In critical circuits the allowable voltage drop in the wire is 0.3V, vs 1.2V for non-critical circuits.
The spreadsheet calculates the allowable resistance based on that voltage drop, and uses the resistivity to calculate a required gauge. It also calculates the required gauge from the ABYC chassis wiring ampacity tables. Then you give it the actual AWG you plan on using, and it tells you how much voltage and how much power you will burn in the wire. It will highlight your selected AWG in red if you violate either the ABYC chassis wire current limit or the allowable voltage drop in the circuit. This lets you make more intelligent decisions about where you want to cut corners.
ABYC (American Boat and Yacht Council) standards are the default in marine wiring, and they translate well to RVs.
Most people just use the ABYC tables. This is generally fine, but they don't account for every situation. In some situations (long runs of bundled wires behind walls) they are justifiably conservative. In other situations (short, high-current bus wires) they end up giving laughably large gauge recommendations.
Don't use this blindly: make a holistic analysis of your wiring diagram, summing currents from the outside in, to make sure you aren't going to overload a circuit. And always ensure that every circuit is fused well below the current limit of your wire.
Hope it's helpful for you.