> Can anyone describe their ideal location for such a socket?

I should think that the "ideal" location for a socket would depend on
where you will be using the device;  From the front seat at the small
table; or the rear seat; or the top bunk etc.

>  but I can't figure out how to route the wire to the "secret"
> compartment from the driver's side battery box.

I have had my kitchen unit and the water reservoir unit out several
times (about to do it again) and I can tell you that it is not hard to
snake some wires in behind the panel (behind the kitchen units).
There is some insulation and bulkheads back in there, but if you take
all of the easy stuff out of the kitchen cabinet and take the panels off
of the 110v cabinets, you will see other wires going in and out of
existing cut-outs of the side panel.

Also my '91 has wires coming out from the aux battery and go in right
under the camping fuses in right behind the kitchen cabinets.  I'd start
there.  If you have trouble fishing a wire around in certain places you
could try and run/fish a piece of (stiffer) small diameter plastic hose
through the tough part and then fish the wire through the hose to your
destination (one can then remove the hose).

If you're looking for a winter project, and if you have never taken out
your cabinets, it might be "fun" to remove all of the cabinets for
several reasons:  1) to clean out 15 years of accumulated crud that is
in back there. 2) to clean the rear of the fridge (cooling fins) & maybe
install new fridge fans, 3) to check for rust (in behind the panel,
especially at the bottom) and to apply rust inhibitors (if you're close to
Canada try Krown).   Directions for most of this is in the archives, not
a hard job.

Any question just ask.  M