I always keep a couple 5 gallon cans of gas handy in my truck's bed, secured with a ratchet strap through the handle so they don't bounce around or topple over. This fuel is mainly for the generator, but I've saved friends letting them use the gas for an emergency top-off.
The most elegant setup are Rotopax installs. These are low-profile, 2-4 gallon containers as mentioned above. My next RV setup will be a motorhome towing a small cargo trailer (4-5 feet width) for my boondocking expeditions. With this, I can mount Rotopax containers on the front of the trailer which will still allow for extended fuel range, but with the use of locking T-handles, the containers will tend to not disappear.