There's LOTS of direct experience to be found on ADVRider dot com
These guys always carry extra gas.
You can also find more experience by searching the interwebs for "TAT" or "Trans America Trail."
There's lots of Forums that have TAT sections or threads, and outfitting for extra gas is pretty common.
I carry oil if I am on a DS ride that has gas stations, but for trails, I have not done any that don't get me back to the truck in 1 tank. ('cept that time I ran outta gas in LPNF, heh heh).
Barry