I consulted with my dealer authorized service people about auxiliary battery
installation. they charge about $100 to $150 to do the work but say they are
happy to draw a diagram for me to do the work myself to save me the money.
How nice.

They also recommend a Dual battery solenoid switch ($12) rather than a
battery isolator which is a more complicated job. Both will charge both
batteries when the van is running and shut off the main battery when the van
is not running.

William