I use an RDS 45 gal only 9" out of the bed full width and height. Gravity feed into stock tank is legal go to Northern Tool they have everything you need and about $550 tank install kit and ebay 12 volt valve in feed line from tank. I have 26 gal stock on the truck and 45 gal. aux gives me an easy 600 miles before having to stop for fuel that way we plan our stops to include best price fuel along the way, The tank has paid for its self in 2 yrs.
http://www.northerntool.com/shop/tools/product_200306745_200306745
http://www.northerntool.com/shop/tools/product_200315501_200315501