In my pickup, between the wheel wells, is a hair over 48 inches. Two 55 gallon plastic drums will sit there nicely on their sides without rolling and if the drums are full or empty I don't get sloshing. I added a hose bib type valve to each barrel's bung to allow filling and draining via a garden hose, and for an air release I used a screw type tire valve with the valve core removed. The barrels are much too heavy to lift when full so I fill them in the truck and empty them from the truck. Whenever water goes in or out, air must replace the lost water through the tire valve. I use the water to gravity feed a pressure washer for bulldozer cleaning.
Barrels are cheap, the valves cost me more than the barrels. A 12 volt, RV style, on demand, water pump can be had for 70$ new and plumbed to spray the water at pressure and flow very similar to a municipal connection. The pump can be powered by making up a socket to fit into your seven pin trailer connection on the bumper of the truck. A standard shower head, pressure washer, kitchen sink, runs about 2.5 gpm and that is less than what a typical RV pump puts out. If you can get 100 gallons out of the two barrels then you are looking at 40 minutes of squirt time.
Water weighs bout 8 lbs per gallon so your truck needs more than 800 lbs of cargo capacity. I find my 1/2 ton truck has absolutely no trouble driving around with the additional weight back there.