Really like the idea of a voltmeter read on the auxiliary battery. You do not want it on continually with the key off, though. (You don't want to drain that battery with a voltmeter that's on continuously.) Since you're using mechanical separation of the two batteries, it's easy to isolate the voltage reading for the second/auxiliary battery.
I suggest using a key-on relay (small Bosch type) to simply switch on the voltmeter for the auxiliary battery. You can use the Bosch relay as an interruption switch (simply a break in one of the wires to the voltmeter in this case). Since the voltmeter just requires voltage and not amperage, all of this can be done with reasonably light gauge wire. Here's a drawing I quickly cooked up for the circuit I'm describing. Let me know whether this works for your aims:
Voltmeter Circuit.jpg 132.06KB
See the footnote in the diagram, I jotted down what you will accomplish with this wiring and a Bosch-type relay approach. Is this useful?