FWIW, we once owned a Dodge B250 van that we converted into a camper van and I wanted long range fuel capacity in it. (I hate stopping to fuel up. Stopping to eat ... that's a whole different matter!)
I found and installed a new after-market 40 gallon heavy gauge steel auxiliary gas tank in the van. The tank was designed to mount up high beside the frame and the body panel. That tank along with the stock tank gave us well over 60 gallons of fuel capacity - for a range averaging around 1000 miles. Fantastic ... I wish our Class C motorhome had that expedition vehicle kind of range!!