Amp gauge question

I disagree. Just because the meter is marked "Alt" and shows a -40 to 40 scale doesn't mean that it actually measures the current directly, i.e. by flowing all of it thru the gage. It could actually sense the dV across the wiring from the cabin to the battery. Indeed, I would be pretty sure they aren't running the full battery current on 18 awg wires. You could also think of it as an ammeter that measures a fraction of the true current (the shunted current). An old gage type voltmeter is actually an ammeter (or galvanometer) with a series resistor.