Northern Tool should have a high turn over for DEF, no mater the brand, more likely the date was not that old. You’re not the first of that phenomenon, as far as the dealer goes. They are looking for profit where they can find it. I had my 3500 serviced by the dealer over the 30K miles I owned it, the costs are minimal to me for them to change oil on a CTD. So I took my new to me 1500 to the same dealer and they used DuraMax oil and a Carquest oil filter, needless to say I now change my own oil on the Hemi. My point is they are tightening their belts. Demand a sample to see if you don’t have an enemy in your your neighborhood. Buy a locking cap and document your service. I did that because of the dreaded CP4.2 HPFP, in case it failed.
Good luck with your truck.
On edit, I recorded all fuel purchase miles and Def fill ups.