Repeated bouts of vomit (with specks of blood) and low appetite in dog.?

I understand your frustration. Unfortunately in veterinary medicine it sometimes comes down to determining what it is not, or ruling things out. It's great that you ran all the blood and fecal tests. If your veterinarian is recommending to test for Addison's (usually done by a blood test called ACTH stimulation) I would do it. Addison's isn't very common, but after trying all the testing and treatments that you already have for your dog that were inconclusive and did not help, it's certainly the next step to see if maybe this is the answer. Usually Addison's is linked to findings on electrolytes (sodium potassium ratio), and as you said there were no significant findings when this test was performed on your dog, but keep in mind that not all cases have all symptoms. Best of luck. I hope your dog gets feeling better!