...I have also always wondered, what is the inflection point. For instance, at what point does the added weight of the additional fuel reduce mpg to the point that it doesn't justify carrying it. I don't have an answer, and have considered testing the theory. Something to think about...
At roughly six pounds per gallon (gas is lighter than water), I don't thing you could have one on any practical level. On the purely theoretical level, towing a
tanker car full of gas
might get you to the point of negative returns if you ignore the towing capacity of the Jeep - which is not going to be possible in a real-world situation. (In other words, an enormous load of gas might drop your MPG to the point where you'd be better off unhitching it and going solo, but you wouldn't be able to move it to begin with.)
I like to break things down into simple terms, so here's a bizarre "what-if": What if you had a 300-pound passenger aboard who could magically extend your range by the equivalent of 50 gallons of gas just by being on board? (300 pounds divided by 6 pounds per gallon equals 50 gallons.) Would it be worth having him along on a cross-country trip? Sure, your MPG would be a bit lower, but the time you'd save refilling would be worth it, not to mention the security of knowing you're not going to run out any time soon after filling up. Take into account the fact that he would be steadily losing weight after every fillup along the way and it's a no-brainer.
I know that during the gas crisis of the 1970s we were all warned not to constantly drive around topped off because of the cumulative effect of the weight of the extra gas, but no one with a long range tank is likely to stay full up all the time anyway. Besides, in those days you bought gas whenever and wherever you had the chance since you never knew when you'd find another open station. (Remember the Odd-Even policy in place for a while?) A guy or gal going far enough from civilization to warrant the extra fuel is in much the same position.
Finally, not to pick any fights, especially since I'm new here, but the fact that someone
wants to do it ought to be enough reason. I got my first Jeep back in May (and am still learning), but the first thing I did was to put a big roof rack on it. I intend to use it after I finish college in a few months, but I mainly got it just because I'd always wanted one. Some people warned me that it would cut my mileage a bit and might be noisy, but balanced against the fact that I just
really like the thing, who cares?