Allergic Dog Scooting - Not sac inflammation?

My dog has been on prednisone all her life, 8 yrs. Her scooting is getting progressively worse. She has food allergies and is always being switched between different high quality food for dogs with allergies. Her skin is often pink and itchy and yeasty. She also has a chronic ear infection. I've tried so many different things and the vet is not much help. Are her scooting problems Candida related or Psorasis or ?.... Please help!