All,
I wanted to follow up with the Vanagon group with the results conclude my
Auxiliary Battery wiring upgrade.

The following is the steps that I took to complete the upgrade.
1.      Run #6 wire from the main to the auxiliary battery for the + leg
only. Drill holes on bottom side of both battery boxes and run wire over top
of gas tank, add in rubber grommets and run flexible plastic tubing to
protect new wire, caulk tubing into place with silicon.
2.      Ground aux battery at spare threaded ground point in compartment
under driver seat (high on right side of battery box). Bolt is 7/16 SAE not
metric size.
3.      Install GoWesty Auxiliary Battery Kit (isolating solenoid) and new
Second Battery Relay 411-915-511B. Upgrade wiring to carry #6 through
GoWesty solenoid to battery.
4.      Install AGM 44 amp hour (AH) battery under driver's seat.
5.      Fuse both sides of the GoWesty isolating solenoid. Solenoid rated at
80 amps per GoWesty folks. Fuses installed in the drivers battery
compartment protect new #6 awg wire at aux battery side with two Blue Seas
MAXI fuse holders and fuses. This should handle any high temperature
conditions that the interconnecting battery wire may see.
6.      Use Blue Seas low voltage subpanel with integral ground to control
interior power, stereo, lights, fridge, sink pump etc. Install 100 amp fuse
inline for overcurrent protection for the sub panel.
7.      Install Battery Tender - Junior or Senior for long term maintenance
of auxiliary battery. Plug into 110 outlet under sink and plug extension
cord into exterior Westy 110 landline connection. I only have the tender
charging the auxiliary battery. If the van is parked a long time then adding
a 2nd tender for the main battery may be useful.

I think that this covers all the basic contingencies of updating the
auxiliary battery wiring as well as having the system properly fused to
handle any potential overcurrent or ground conditions.

Also I found that Interstate makes great "cold weld" battery cables that
have the glue filled heat shrink water proof connections. I could not have
done the work without these excellent cables.

Thanks for all of the great advice that I received from the Vanagon list!

Kevin