I too have struggled with coming up with the best way to carry water in the truck. The best solution I have found is the Front Runner's footwell tank, however, since my kid is big enough to need the seat-to-footwell height I have not actually purchased one. My second best solution has been to use the Scepter 5gal container and lay it down flat on the other footwell (low center of gravity and unused space) in the rear passenger area. I have yet to come up with a good way of securing it in case of an accident, rollover etc but that's on my to-do list. Between that, a 100oz Camelback hanging from the headrest, two 32oz Nalegene's and a gallon of Arrowhead jug in the back (7.5gals water total plus Gatorades etc in the fridge) we've had enough water for 3+1 dog for 3 days comfortably.
