Neil,
  The way I did it in my 84 when I added the interior and I needed power
back there was:   I took it from the spare battery direct.  The spare
battery resides under the drivers seat.  I ran a big fat line off the
battery with a big fuse right there at the battery,  in the compartment.
Ran that fused supply line through a hole drilled into the pedestal of the
seat so that it ended up inside the cabinet under the sink.  Then, on the
front wall of that cabinet inside, I wired in a circuit breaker/fuse block
that I got from one of the racer supply houses online.  Has about 6 fused
circuits available for all the powered camping stuff and power outlets that
I have in my camper.  Makes them all run off the Aux battery (Isolated from
the  vehicle batt by a big relay) so I don't wake up to a flat starter
battery.  Each of those circuits, from the 'gang-terminal' is fused at that
block, too..