I would suggest a board certified vet nutritionist. You can find one at
American College of Veterinary Nutrition
You don't have to physically see them. You can have your vet contact them, or you can. They do need to see records.
They have a wealth of knowledge. I have done consults on a few of my sick pups and some are on RX foods, some on home cooking. There is one down here in Houston who I have been very happy with. Her website is relatively new but her contact information is there
www.lenoxnutrition.com