For a medium duty truck, try using tanks from a Western Star tractor. They're either 120 or 150 gallon, but long, and narrow. Should be up off the ground a little more than others if you mount thm correctly. I get most of my used stuff from either 4 State Trucks in Joplin, MO
www.4statetrucks.com or local in Grand Rapids, MI from Frontier Truck Parts
www.frontiertruckparts.com
4 State seems to hve a better selection, but Frontier is alot closer to me.
They currently have a '94 Western Star tank that is 150 gallon, measures 22" tall and 96" long, with brackets and steps for $450.
IT's a right side tank, but the left side is there too, you'd just want to take the paint off the aluminum on that one.