Rhombus,
I don't know that I would recommend using the spare tire well for a battery. I have put my truck on top of the spare tire so many times it would make your head spin. I can see using it for an air tank if you relocated your spare, but I don't think a battery would survive if you put all 4,000 lbs of Xterra on top of it. Just a thought.
The Warmonger