RE: Interior Lighting
Although I haven't moved all my interior lights to the aux battery,
the best solution I have seen is to make the change at the fuse block
under the dash.  All the interior lights and radio run off fuse 3. 
Remove the fuse an run a fused line from the aux battery to the load
side of the fuse bracket, this will move the interior accessories off
the starter battery.
As for adding lights, when I added lighting above the front seats I
pulled the headliner loose and tapped into the wiring above the
drivers door, the line that runs back inside the tray above the
sink/stove.  And for the rear light I added above the rear seat/bunk,
I pulled that tray loose and tapped into the line at the stock light
and ran a pair of wires from the back end of the tray to the center of
the headliner (in my '85 w/o AC) or in the case of my current '90 with
AC into the AC plenum to that light.
Hope this helps. It's not complicated, but does require pulling a
bunch a little screws and some sneaking of wires.