http://www.ahoycaptain.com/plastimo_water_tanks.html
I have the 26gal. The tank is out of the truck for winter, but I can snap some pics of the two hoses (fill and use) I attached to it if you are interested. I built a pair of simple 2x4 and plywood boxes to fill the rear foot wells and level the floor of the XJ, then set the tank on top of them tying it to some anchor points. We then tossed all our soft gear (back packs, sleeping bags, pillows, etc) on top.
I am currently building a steel structure in the KJ that will form a shelf above the rear seat area/foot well for the fidge in place of the 1/3 seat, with the 2/3 seat mounted in its normal position (just no leg room) on top of the structure. This will open the entire area from the load deck to the backs of the front seats, and rocker to rocker, for the tank to occupy.
I did it to keep the weight as low as possible, while the use of the bladder prevents any sloshing back and forth on those rocking trails.