I had a 115 gal. alum. diamondplate, in bed, auxillary tank installed by Dually Depot in Elkhart, IN. They sell a lot of tanks to the guys that haul trailers across the country, commercially. Total capacity, on board, is approx. 150 gals. , now. You can go a LONG way before needing a fill-up.
The set-up is kind of neat. There is a six-way valve (elec. ) & the trucks factory gas guage tells the level of both tanks by using a rocker switch neatly placed in the dash.
Joe F. (Buffalo)