Hello,
I am sorry to hear about the skin problems. I have a Cocker myself.
You have certainly been through quite a few treatments. It certainly sounds like a primary allergy problem.
Few things:
1. Food allergy. To really assess if food is the culprit, we need to change the protein and carbohydrate entirely. Most foods contain beef, lamb, or chicen and most foods contain rice or corn. So, you need to do a diet like Vension and Potato, Duck and Potato, or Fish and Potato. You also need to do it for at least 3 months to assess any improvement. The most popular prescription food for this is Science Diet D/D which comes in all 3 flavors.
By the way, when you start, you cannot give a kibble of anything else. Nothing else other than the food.
2. Atopy or Environmental allergies. You can actually do a blood allergy test called the VARL panel which can help you identify if there are bad allergies to things like weeds, grasses, trees, etc..
With these results, your pet can actually get allergy injections, just like a person, where you learn how to give the desensitizing injections yourself.
3. Treatments
No question, you are going to need some steroids to help out this skin and get some initial relief. You should also consider a medication called "Atopica" or cyclosporine. Excellent medication that affects the immune system response to allergies and works wonderfully. Don't forget, if there are any active infections, which there usually are on the skin, your vet should prescribe an oral antibiotic and possibly antiyeast medication. Love the Douxo shampoo is a complement to other treatments.
That should get you off to a good start
Hope that was helpful
Please let me know if I can be of any further assistance.