I wouldn't be without my auxiliary tank!! Pure and simple.
It's only a 40 gallon tank but that gives me the flexibility to stop for fuel when I want to stop and not when the fuel tank dictates. I usually fill up in the evenings after we're stopped and unhooked, so I don't have to find a station that my 40' toyhauler will fit in easily.
Personally I wouldn't want a 100 gallon auxiliary tank, just too much weight for my truck, and I don't need to be hauling around an extra 300 pounds of dead weight all the time. I'm cheap so the tank is from the Farm Store and the electric fuel pump/nozzle was off Craig's List. Total amount spent was about $550. I have to manually pump the fuel into the main tank but that is really no big deal, IMO anyway.
But another few gallons so you can stop at a rest area instead of a smelly gas stop is another plus to the auxiliary tank.