I am so sorry for you. I remember well when I lost my precious "little girl," a chihuahua I'd had 10 years, one day before my 44th birthday. She died of renal failure, secondary to heart disease. I had to have her put to sleep, because she was suffering, much as your dog was. (Begging for the food in her dish that she could not stand to eat, ditto the water, weak, weight loss, etc.) Yes, there is some overlap in the symptomology of pancreatitis versus complete renal failure, but only superficially. The mechanics behind it are different.
But I know the pain.
The next day, my dear sweet loving (late/deceased) sister came to visit me, to sing "Happy birthday" to me. I paused, cried like a 2 year old, "I miss Chi-Chi!" and she bawled, too. Both of us crying like kids.
Do I think, given what they've said about the pet food problem, that it could have been the pet food? Very possibly.
The only other question is this: did your dog ever, even accidentally, get access to human food? A good number of human foods, like chocolate, can cause pancreatitis, and kill dogs. Most of what we eat cannot hurt them, but the list of what can is surprising. (No, I don't know the full list. My vet and I were discussing it, once.)
Oh, just FYI: why do I know about pancreatitis and renal failure, etc., like that? I was a RN for 24 years before my lungs gave out and I became disabled with oxygen 24/7. Most of the principles over lap. Often, if you take time to look at it, they even use some, not all, "human" medicines, for dogs, cats. etc. It's not uncommon for elderly cats or dogs to get Lasix or Lanoxin, for instance.