Let me put in a pitch for the Yandina battery combiner.
http://www.yandina.com/NewCatalog.htm
then click on the C100.

It is very small, very slick.  The wires that connect
the two batteries are not huge, so it limits the charge
that crosses through the Yandina, but that has been
no problem for me.  If I drive a couple of hours, my
aux (122 Ahr) is charged.

I have a battery switch with big wires, but I almost never
switch it to "both" anymore.  I put my Yandina next to the
switch, hooked the leads into the posts of the switch,
rapped the wire around inside the switch case and was
done.  (You cannot shorten the two power leads of the
Yandina--the length is part of the current limiting design.
They are about 18" long, as I remember.

Richard