There is another solution to increasing aux battery capacity that hasn't
been mentioned in the various threads that have been running the last few
days.  Add a THIRD battery in the battery compartment/space in the engine
compartment. the compartment/space that was/is used for the diesels.  I know
of one such installation that has been trouble free for several years.  That
setup uses two Optima Yellow Top batteries, one under the drivers seat and
the other back in the engine compartment, wired in parallel.  This gives a
theoretical 110 AH of !2v 'juice'.  It does require running a couple of
really big wires the length of the vehicle but makes for a really neat
installation.  And not a bit of storage space used up under the rear seat.
BTW, don't ask me for details on this because it's now my install and I
haven't looked at the nitty gritty details on where the cables were run or
how this was done.  If the owner of the setup I'm talking about (who will
probably read this) wants to volunteer info that's up to him.