if youre not happy about the gas tank size there is an aftermarket tank out there. Its another 19 gallons. not sure on cost or installation. Uphill has it on the Scorpion (sick rig) and from what Ive read he seems to really like it. check out the thread.
http://www.fjcruiserforums.com/forums/man-fre/54845-fj-cruiser-auxiliary-fuel-tank-installed.html
http://www.fjcruiserforums.com/forums/man-fre/54845-fj-cruiser-auxiliary-fuel-tank-installed.html