Sounds similar to what our old poodle had when he got to his senior years-around 11 or 12. He was scabby, flakey, had lots huge patches of hair missing, and all the hair on his tail fell out. He was a mess. And he smelled awful. After many tests (including cancer tests and allergy screening), lots of money, lots of drugs, it turned out to be his thyroid that was causing most of his problem. (which had been tested before, by the way) The vet put him on a dose of medication for the thyroid, and it helped him quite a bit. He never did totally recover from the condition, however. It was something we just had to manage. He required baths in a special treatment shampoo, and an Rx salve for his scabbies when he'd scratch open a sore (which happened less frequently after the thyroid meds). Sometimes he'd need to be on a course of antibiotics, because his sores would get infected. We also gave him benedryl and a low dose aspirin occasionally, on vet's recommendation, just to make him feel better. But he never seemed to mind any of it. He lived till he was 16. Don't know if this helps. But I can say, it sure was a maddening process trying to figure out what was wrong with him. Best of luck.