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That hasn't been my experience in the Pacific Northwest, northern Cali, and Idaho. I can look around a little and find name-brand diesel up to 20 cents/gal cheaper than truck stops. For example, for several months in Orland, CA the Shell station was ~$2.65 and the Pilot next to it was almost $3. (I think She'll has somewhat "caught up" now...). In southern Oregon it's easy to find name-brands ~$2.60-2.70, and the truck stops are $2.85 plus. It's about the same in northeast Oregon.
I've also run into freeway interchanges where all of the stations cluster around it would be 20 cents or so higher or lower than anywhere else in the area. As of last week, for instance, I-5 exit 19 in Oregon is an example of a "high priced interchange" and Hwy 99 exit 205 in northern Cali is an example of an area that seems to have lower prices.
Somewhere in my 65 years I've leaned not to take very much for granted. . Keep yer eyes open and yer hand on yer wallet!
Pricing must depend on where you live. My area is like yours, where truck stop gas costs more. It's not about taxes, it's just what they charge. Anyway, you can probably fill up ten times with diesel before going to the truck stop and getting diesel+DEF at the pump. Honestly, it's pointless to fill DEF every time because it only uses about a quart per tank, in my experience. I have a fuelman card just for these occasions.