Per Ford's Web site, the standard fuel capacity for a Ford F-350 Super Duty (gas or diesel) is 30.5 gallons in a shortbed and 38 gallons for longbed models. No mention that I … can find of an optional second tank.
Ford F350 Towing Capacity >> New 2017 F Series Super Duty Details Released Fleet Owner

Ford F350 Towing Capacity >> New 2017 F Series Super Duty Details Released Fleet Owner

recommended for trailer towing: C-MAX Hybrid, Fiesta, Focus, Fusion Hybrid, Fusion Energi, ... F-350/F-450 DRW Super Duty Pickups F-350/F-450/F-550 Super Duty Chassis Cabs ... power and towing capacity make F-150 the preferred choice for towing and hauling.
Ford F350 Towing Capacity >> 2017 Ford Super Duty Overtakes Ram 3500 As Towing Champ

Ford F350 Towing Capacity >> 2017 Ford Super Duty Overtakes Ram 3500 As Towing Champ