For whatever reason Ford puts smaller gas tanks than its rivals on its pickup trucks. A GM 3500 has a 136 liter tank. For towing a larger tank is a blessing and doubly so if you have a diesel engine. The only downside is that you need to either be more cautious in terms of aging fuel or use a stabilizer additive.
Check which aftermarket tanks are available from Titan. Their tanks are expensive but a nice option if desired/needed. The tanks are specific to a make and model and also a bed length and cab type. For example they make them for a GM truck with a double cab or a crew cab but not an extended/double cab. The devil is in the details.
With a Titan you replace the factory tank with a larger one which to me is a better approach than adding a gravity tank in the bed of the truck and the cost will be very nearly the same either way. The exposed tank in the bed needs heavier duty construction and a separate pump so this adds to the cost.
Range without a load and range when towing a heavy load are very different. My truck has a 36 gallon/188 liter tank and that provides a range of roughly 600 miles with no load on the open highway. Towing or hauling that drops down to a range of roughly 300 to 360 miles depending on the load, the amount of wind, and the terrain.