I have a 105 gallon auxiliary tank in my truck. It makes a world of difference not only in range, but I can now take my time when shopping for diesel so that I can get the best price. For every penny cheaper I can find fuel, I save $1.00 on the total fill up. It really pays to search for the cheapest fuel. When we go to the beaches in California, I can top up everything and not have to pay the state's huge fuel prices.
When towing, my truck gets about 8 mpg leaving me with about 220 miles of usable range. With the auxiliary tank, I can push it to the limit, pull over to transfer fuel and stretch legs, then get back on the road. Over a day of driving, this can save me 20-30 minutes.