I'm planning to post my installation report for my own aux. battery,
but in the mean time I thought I'd mention an alternative to
battery relays and isolators that I hadn't seen mentioned yet: a
"battery combiner".  They're available from West Marine
(www.westmarine.com, but the print catalog is much better than
the web site) for about $90, and - aside from the price -
they seem to be an ideal solution.

The combiner is about a 4 inch cube, and has 3 terminals on it -
battery 1, battery 2, and ground.  It senses when either battery is
charging - that is, is receiving an externally-supplied voltage.
If one is charging, it connects the two batteries, so that they
both charge.  If neither is charging, it keeps them isolated.
Note that this works regardless of the charging source -
for example, if you're charging either of the batteries from
a 110V charger, then the other will charge too.  (I may have
the details slightly off here - it might wait until one battery is
fully charged before conencting the other - but this is the
basic idea.)

The wiring and operation are simple.  No connection to the
alternator (or anything else except the batteries and ground)
is needed.  The combiner has an internal circuit breaker;
additional external circuit breakers are reccomended, but not
required, depending on your wiring and paranoia level.

Personally, I ended up using a battery relay ($12.95 from the
local RV store), to save money.  I came pretty close to choosing
the combiner, though, and I'd definitely choose it over an isolator
(since the price difference is less, and isolators are more complex
to hook up and seem more prone to subtle problems).

                                     -Steven Sittser