I know Mel has one battery on his moose truck, maybe he will chime in.
Correct. I have a single group 31 under the bed, right in front of the left rear wheel. I welded up a tray out of angle steel, and bolted it to some fabricated brackets. There is a hold down of the typical plastic and threaded rod variety. During the 2015 IDI Weekend, when we did the parade, I discovered that my battery was not cutting it for parade duty. Extended idling while in the parade, combined with electric fans, did not make for a good combination. Otherwise, I have had no problems. Of course, remember that the Moose Truck is a garage queen, and does not see severe duty or much cold weather operation. That said, I would definitely recommend remote mounting of the batteries. The reduced weight on the core support definitely will not hurt anything, and the extra space it creates behind the headlights can be used to great advantage for a variety of nice modifications or accessories.