*Question: What is gained by adding an aux battery and ONLY putting in the
relay that connects the two batteries while the engine is running and
charging?  Installed like the wiring diagram in Bentley? And no other wiring
changes*.

I'm asking because of a question to me by pmail about adding an aux battery
after I posted the message yesterday about a good source for Trojan
batteries.  It dawned on me while composing the answer I sent that one of
two things were true (probably both): First, there is no gain at all because
the aux battery isn't connected to anything but the 'fridge and then only
when the engine is running.  Or Two, I've missed something entirely in
understanding the basic aux battery install instructions and wiring.
I DO understand what has to be done to fully utilize an aux battery.  I'm
running a Group 31 100ah AGM battery with a 40 amp three stage charger and
100 watts of solar panels through an MPPT controller in my '90 Westy with
fuse #3 rewired so that everything runs off the aux battery.  My question is
only about the basic, simple, add one relay and do nothing else install of
an aux battery.