RE: You could, conceivably, connect all the extra Westy stuff (interior
lights, fridge, etc) to the main battery and it would work.

Actually, the Westy comes from the factory with everything on the starting
battery.  If an aux battery is added using the stock relay setup ONLY the
refrigerator is moved to the aux battery.  All the interior lights, radio,
and cigarette lighter remain on the starting battery UNLESS additional
wiring changes are made, changes not included in any of the usual
instructions for installing an aux battery whether with relay, isolator or
separator.  Most setups add additional 12v outlets that are attached to the
aux battery so that the computers, DVD players, etc.  only draw from the
aux.

The instructions for moving the interior lights, etc to the aux battery have
been post several times recently, check the archives.  There is a difference
in how it is done depending on the year.