Looking at shunt based voltage meter gauges too monitor both batteries (state of charge, current draw, voltage, etc) and am leaning towards this 350A round gauge
It is 2 1/16" and will fit in an A pillar gauge mount, either in an AutoMeter aingle
15411 Gauge Mount, A-Pillar Pod, Single, 2 1/16", Toyota Tacoma 95-00/4Runner 96-00
www.autometer.com
This one is black and will need to be painted to match the interior
Or a Lotek dual gauge pod already oak colored to match my interior, but requires removing the a pillar grab handle
GaugePods.com is your go to place to get custom pillar pods for your G-Body, Mazda, Ford and everything in between.
www.gaugepods.com