[anonymously]

Try malaseb shampoo, or any gentle shampoo (only) for now, twice a week if you notice it helps. Don’t put anything else on the skin as it may cause irritation and make it worse and don’t make diet changes until you see the vet and he approves.
Her symptoms may have nothing to do with the food.
Food sensitivies (food allergies are rare) usually result in GI disturbances such as vomiting and diarrhea, not pruritus (itching) and rashes.

I have a dog with environmental allergies she responded to treatment from a dermatologist and is stable now, she can eat anything but does best on Nutrisca Salmon and Chickpea dry a base.
She receives Allergen Specific Immunotherapy. It’s the only thing that helped.

So work with your vet, he may have prescribe meds to get things under control, if you don’t see improvement within a month or two and the dog appears to be suffering, consider seeing a specialist for skin testing. The vet can do blood testing, but the skin testing is the most accurate for environmental allergies.

Don’t be fooled by mail-in saliva and hair tests, they are not accurate. People complain that their dogs test positive for everything!

Do not give any over the counter medication…or any medication unless it is prescribed by a veterinarian.

Also, you can use the search engine here: “allergies” and “anal glands”