You could just get a plastic 55 gal drum, you'll usually find some on your local craigslist. Probably around $20-30 each. You can find one that held innocuous contents tho washing one out 4 or 5 times will generally make them safe to use. The EPA standard is 'triple-wash'. Be aware that 55 gal of water weighs almost 500 pounds so you will be adding a big load to your truck. That comes right off your payload. I'd lay it on it's side with the large bunghole at the 12 o'clock position. Fill it thru that. You'd have to firmly chock the drum since you will not want it to roll around.
Just looked at my local Craiglist and saw this:
"Plastic Barrels:
10 Gallon Wide Mouth Food Grade $12.00 Each
15 Gallon $10.00 Each
30 Gallon $15.00 Buy 4 Get 1 Free
55 Gallon Food Grade $15.00 Each Buy 5 Get 1 Free
55 Gallon Organic Food Grade $25.00 Each
55 Gallon Wide Mouth Food Grade Brown Barrel With screw on lid $30.00 Each Or 4 For $95.00
60 Gallon Wide Mouth Food Grade Brown Barrel With screw on lid $30.00 Each"
I'd really look at 30 gal drums tho.
