"How does one go about storing extra gasoline in the back of an SUV?"

Never. Fumes aside, It's far too dangerous, for the reasons mentioned in the event of a collision from explosion or a filled can hurtling through the cabin. Even if you weren't somehow injured, it's hard to imagine a plaintiff's lawyer not having a field day if someone else in the vehicle was. Roof-mounting a DOT-approved can, preferably a NATO steel model with cam-operated locking top, is probably safest. It is non-vented and expands with temperature (and is built to withstand this, the Afrika Korps and later the Allies used them in 130-degree Saharan summers with no problems). The best mount is on a rear frame- or a roof-mounted steel rack (tends to crack and break fewer parts with the constant pounding off-pavement), with the can secured into a heavy-gauge G.I. (not aftermarket) steel can holder - I've noticed leaks with all other types of containers when used on an SUV off-road. The roof mount is probably safer than the rear frame-mount, but there is still a risk when installing and removing filled cans. I also cushioned the bottom of the can in its fitted GI holder to prevent metal-to-metal wear (incidentally, plastic tanks stored without special pouch-type holders or plenty of protective padding on roof racks will wear through even faster off-pavement). You'll need a lot more than a bungee cord in the event of hard bumps or a collision.

However, even using exterior-mounted gasoline containers is a fairly dangerous activity when dismounting, decanting, and refilling them - just watch your friends back away from you when you begin. A rear-end collision, a spark from dropping a can while taking it from the roof - lots of bad possibilities there. Never fill the can until just before heading off-pavement, and do not drive for any distance on roads shared by other vehicles.

Better solution is a larger, heavy-duty fixed tank in the stock location. The best solution? Probably a fuel-efficient diesel-engined vehicle with large factory tanks, which is what I eventually decided upon.