If you are going to spend much time up and down the Rockies you are going to burn lots of fuel. We were just there in June and had a great trip but lots of up and down the whole month. I don't know of a legal (at least here in Calif) bed tank you can get for gas trucks, diesel is another matter. You could put a larger replacement tank from Titan or one of the other companies that will give you an additional 10 or 15 gallons of capacity. In any event I would suggest carrying a couple of 5 gallon cans for added insurance.
We have a diesel and a 37 gallon in bed aux tank for a total capacity of 66 galons, It's great to not have to stop for fuel but once a day if that. We do stop every couple of hours to stretch the legs and let the dog do his business but to have to fuel that often would be a real pain in the back side.
Have fun on your trip, Yellowstone is an amazing place.
Steve