This won't help as much as a dedicated aux tank, but I'd consider some way of mounting
Rotopax fuel cans on your rig. They can be mounted on the roof, on the side, on a hitch mounted cargo carrier, or anywhere there is a solid surface. With a locking T-handle, the fuel cans are protected from two legged varmints as well.
Gasoline cans come in the thicker three gallon fuel packs, diesel only comes in 2 gallon containers, but they can be stacked, and you can buy the "double-sized" four gallon containers if needed as well (although they take two pack mounts/T-handles.)
I am personally looking at getting a set of these on a hitch mounted cargo rack, or if I get a fairly narrow trailer, mount them securely on the sides. With a gasser, four three gallon containers can get me at least 100-200 miles more range and if boondocking, an additional 24-48 hours of onboard genset run time.
Of course, Rotopax are expensive, but if mounted right, they will stay put.