Why is my senior dog losing weight, has diarrhea, and possible cancer?

I'm sorry Diesel isn't doing well! If he had pancreatitis, you would see symptoms such as repeated vomiting, vomiting undigested food, a hunched pain, a painful abdomen, a distended abdomen, diarrhea, loss of appetite, dehydration, and lethargy. Diesel would be extremely ill if he had pancreatitis, and from what you are describing, I would be inclined to think he doesn't have it. It is possible that he does have cancer even if your vet didn't feel any masses in his abdomen. There can still be tumors even if they can't be palpated, and cancer would cause him to eat and lose weight. Cancer can also cause diarrhea. Certain cancers also do not show any changes in blood work. I recommend having further testing done on him, such as x-rays or an ultrasound of his abdomen. These would help visualize if he has any tumors in his abdomen, and further testing such as a biopsy of them can be done if any are found. Unfortunately cancer is very common in senior dogs. I recommend having him seen by your vet again to discuss further testing. I hope this helps!