Here's another wrinkle in the whole equation (to mix a metaphor): the
voltage regulator/alternator system doesn't seem to be adequate to keep the
batteries charged.

Under load (fridge, AC, headlights, etc. on) my system barely generates 12.3
volts at the vehicle battery, and about 11.8 at the aux battery. This is
with a rebuilt alternator and new vehicle battery (a week old on both.) With
no load, voltage is normal, about 13.8- 14 volts.

In other words, there's not enough voltage to charge the batteries, no
matter how you configure them, while driving.

Those of you who have experimented with variable voltage regulators, what
have your results been? There seems to be consensus that Bosch regulators
are "lazy" when it comes to dealing with load, and that's what I'm seeing.

If you're using a battery charger on the bench (or bedroom floor) to test a
system, it doesn't seem like you'd get the same results as in the real
world, where your alternator is trying to power umpteen things and charge
two batteries at the same time.

Maybe we could eat that cake at a free lunch, to mix another metaphor.