It costs me $70 to fill up my daily driver. But that gets me 600-700 miles down the road and I do it about 3x a month. $/fillup is not a useful measure. Filling up a diesel truck will cost you more per tankful most of the year. Now, it's about 20%-25% higher, so it will be $120-125 per fillup. Will you travel more miles per tank? probably. Will a diesel cost more to purchase than an otherwise similarly equipped and condition gassser? Yes. Will you notice and appreciate the difference in towing experience? Absolutely. Should you spend an extra $8k for a new diesel vs. gas? No idea.
If you want us to help you, you'll have to provide more information. How many miles per year towing vs. not? What is your current truck and does it not perform (besides fuel cost) to your expectations? Do you want to replace it anyway if is it just curiosity and annoyance about those $100 fillups?