Dan, yes, what you describe is exactly how it would work. There is a 0.2mA continuous current draw from the AUX 'input' which is partly just the current draw for the sensing potential divider circuit on the board. That 0.2mA should not cause an issue with battery charge balance given there may be that much leakage just from battery posts and dirty battery case conduction paths...George can we revisit the 24v bank usage. If I have digested correctly everything you have outlined I think it sums up something like this:
Pull the diode out.
Ground the gauge to the chassis.
Feed the gauge aux + leg with a center-tap.
Feed the gauge main + leg with switched 24v bank power.
Powered up the "main" reads total bank and the "aux" reads the lower battery.
I understood you to say that there is a continuous, albeit slight, current draw on the aux battery. Is that still the case here?