Not a bad idea. I have several gallons of water under the back seat currently in 24oz bottles, and there is definitely space that could be used more efficiently. Downside is that, unlike a sport bike, a couple of 32oz bottles wouldn't get me very far... That's only about 18 miles in my Xterra and I would really like to extend the range by about 100 miles. I don't think it would be worth the hassle to have 10 32oz bottles stashed around the truck that I have to rotate every 6 months.
Thanks for the suggestions so far, you guys have presented some really neat ideas that I might end up pursuing.