I must say that I am really not a fan of vet prescription foods although I do realize they have a place in treatment of some serious conditions.
If you are looking to treat allergies then the first step I would recommend is to list all of the foods that you have tried and then try to identify common ingredients. Basically all the ingredients up to and including the first listed fat source would be what to compare. After you have identified that list then try to find a food that does not have those ingredients.
Other choices include trying a raw food diet. Many dogs that are allergic to processed kibble proteins/grains/fats do not exhibit any of those same allergic reactions in a real food raw diet. My yellow lab
had horrible allergies but could eat those same foods (chicken etc) easily on a raw food diet.
Good luck.