There is the "Law of Diminishing Returns" at work here. If you have to fill the DEF tank every 6100 miles (I have not seen that to be the case but) then in 150,000, you will fill it 25 times. Since it's a 5 gal tank, you will probably never want to put 5 gals back in it (much like your fuel tank) but for the sake of discussion, let's assume 5 gals.
That means you will use 125 gals every 150,000 miles so for every $1 difference in the cost per gallon, that's $125 spent or saved. I suppose it depends upon how many miles you drive a year and how long it takes you to put 150,000 miles on the truck, but $125 delta operating costs on a $65,000 truck seems to be a "nit" compared to the cost of Fuel, Oil, Tires, Maintenance Labor, etc.
Assuming you get 18 mpg, you will use 8,333 gals of fuel driving 150,000. Better to save the fuel cost by not having to hunt the cheapest DEF since that works out to requiring 1 1/2 cents savings on fuel cost during the same period.
Makes you really consider what is important when you look at all the inputs and outputs.
BTW - Pilot, Loves, and several other Truck Stops in the Houston/ Austin area sell bulk DEF.