I installed a 35 gallon tank in the bed of my truck that I purchased at Tractor Supply for $299. It is steel and has two bungs, one on each side that I secure with pad locks. It is the same width and height as the truck bed and only ten inches front to back. I have a full size Tractor Supply tool box mounted between the tank and 5th wheel hitch (did mention that I have a long bed?). The tank is plumbed like all other gravity feed tanks with an adapter I purchased at Northern Tools for $60. Because I was concerned about dirt and water infiltrating the auxiliary tank, I installed an in line WIX diesel fuel/water separator filter and a low pressure diesel fuel pump toggled to the drivers cab, adding another $100. Total investment is less than $500. and it works great. Pulled my fiver from Nashville to Kansas City before I needed fuel.