Here is one 12V chart:
http://www.offroaders.com/technical/12-volt-wiring-tech-gauge-to-amps/
and here is another:
https://www.crutchfield.com/S-DgFaM8UxNNs/learn/learningcenter/car/cable_gauge_chart.html
Most charts don't go down below 10 or 15 feet but I found these two that do.
I think we are all in agreement. Wire gauge and length should be sized to the max amperage it will be be expected to carry. The connecting wires between banked batteries will be short and so can be higher gauge and carry the same amps that the longer wires carry from the trailer to the battery. Those of us with OEM electrical systems will do just fine with the short premade connecting wires available from the auto parts store.
Thanks for checking my thinking. All my electrical training came from a physics course many years ago and there was not much real world application so I always check with people who know more than I do.
