Vincent, many of the Sportsmobile’s have a tank holding ruffly 38 to 48 gallons - depending on the ‘build’ (about 144 to 180+ liters).
When you calculate MPG on a 4x4 Van weighing (again depending on the extent of your build) around 9,000+ lbs your fuel mileage probably will not exceed 14mpg (taking into consideration type of terrane, you could be closer to 10mpg). The size of your tank becomes the yardstick for the travel distance you envision.
EDIT: I believe I may overlooked your intent; Are you leaving the original fuel tank in place and fabricating an auxiliary tank so you have more available fuel?