I'm running two Deep cycles and just drove from Vancouver to Seattle in my Previa. Batts were at 60% DOD when I got there. This included about twenty starts in the border line up (45 minutes of idling saved) They are used series 27 batteries @ $50 ea. from a local shop.
Another thing to remember is that your alternator is also charging your battery. It has to recharge each time you start and trickle charges constantly. I have never seen a battery take less than 3 amps after sitting overnight and it would be much more after you just pulled 400-500 amps to get it going. All this at a time when engine is cold and running rich.
Now that my Previa is alternator free it starts easier , has more pep and revs quicker. Not to mention getting 26 mpg highway (20mpg EPA)
Read what the experts have to say about alternator loads:
IEEE Spectrum: Automotive Electrical Systems Circa 2005
Cut that gas hog out of there.
P.S. Guess who was instrumental in getting DRLs into cars? (Hint: EV killers)
G.M. *******s.