On Vanagon Westie's with the 3-way dometic refrigerator, there is a relay that
disconnects the 12v power to the thing when the alternator is not operating --
you can't actually run one of the stock set-ups off the battery on the 12v
setting, only off of the alternator.  So you won't discharge the battery.

Makes it kind of silly to add in the second battery, since it doesn't run 
the really big load of the 'fridge.  In any event, there is a relay on top of
the refrigerator that turns on and off with the alternator -- kind of handy
if you want to power some really big load only while the engine is running.

malcolm

>      Why doesn't your battery discharge when you leave your refrigerator on 
>      for months?
>      
>      That's one hell of a deep cycle battery you got there.
>      
> I know this contradicts other advice you have received, but this is my 
> experience... I nearly always forget to turn it off, sometimes leave it 
> on for months.  No harm done.
>