Additional fuel carrying capacity on pickup

For an absolute greenhorn the first owner of our truck had it well prepped from new. 2006 Chevy dually, long bed so 35 gal? stock tank. He did one better. He had a split 30gal/toolbox put in the back. Two valves on the floor infront of drivers seat to select draw & return for the tanks

500 more or less flat land towing between fill ups is easy. No stress when travelling to Alaska where stations can be well spread out in N Canada.