Holly I have spent a fortune on skin scrappings, medication, washes and tried so many different diets for a girl I have here who developed an allergy to something in the environment here 2 years ago. She was almost furless and her scratching and chewing drove me mad, plus she was feeling pretty lousy too.
She was being fed a BARF diet.
Finished up doing a lengthy and strict process of elimination, diet and environmental wise, and chicken of any sort, and kikiyu grass, (compliments of a chance remark by Dr Harry) and which is everywhere here except the front yard. Juliet now resides permanently in the front yard and verandah, and diet is Eagle Pack Holistic Lamb with tinned whole fish (cat food) with vegies and yoghurt and she is looking beautiful. Just quietly she has the occasional turkey steak also.
Good luck, I know how frustrating it can be.