Any allergies can change over time, I was deathly allergic to milk as a baby, and until I was around 10 years old, now, 27 years later, I can enjoy a milkshake, followed by an ice cream cone, followed by a sundae. Similarly, my sister was deathly allergic to bees, but she was stung a year ago and didn't have her kit readily available, and she survived. So as far as it being the food, time will change things, and maybe it's the food, maybe it's not.
My parents had a Newfie with skin problems, and they did all the steroid shots, nothing seemed to work, they finally took her to a vet that specialized in allergies, and found out that, of all things, the dog was allergic to grass. It meant major changes in the way the dog's life was led, but, ultimately the dog was itch free and happy, even though her "yard" ended up being the blacktopped driveway.
On the other hand, I had a friend with a GSD, and he was pulling all of his hair out, his vet gave him all the shots, and medications, and put him on the Science Diet allergy food. All told, it was costing him well over $100 a month to take care of his Shep, and the dog still wasn't doing all that great. I bugged him a little and he ended up going against the advice of his vet, and feeding Canidae instead of the really expensive Prescription Science diet. A few months later, his dog wasn't on any drugs at all, and was perfectly happy. I offered him that advice after talking to my vet at that time.
My advice is, if your dog's having problems, and you see your vet about those problems, and they don't improve, see another vet. If it was you having a problem, and your doctor couldn't fix it, you'd see a different doctor, wouldn't you? It's not that one vet is better than another, or one doctor is better than another. They all just have different insights. Your current vet might be the best person on earth to take your dog to if she breaks a leg, or has a belly ache, he may, however, not be the best one to deal with an allergy.
My wife and I have several dogs, and several horses. We also have two different dog vets, and two different equine vets. We got into that habit just for the sheer idea that if there's an emergency and we can't get ahold of our "normal" vet, we could still get ahold of another vet that knew our animals. All of our vets know that they're not the only one, so it eliminates the chance that one may feel "jealous." We make it clear that we're perfectly happy with the care each of them shows to our animals. We also make it clear, that for the well being of our animals, we like keeping our options open.
Doctors, be they human doctors, or vets, are still only humans, they're not all knowing, they're not gods, they make mistakes, they miss things, they have preconceived notions. Seeing a second one is not an insult to a vet you may love, and if they're truly a caring vet, which most are, they'll understand, but more importantly, if you see a different vet, and you do find the answers you're looking for, DON'T take it out on your current vet. Tell him or her what you found out, ask him or her for their assistance in treating it. Make them part of the solution, don't blame them for the problem, and it will make them a better vet, and most importantly, it will make your dog happier.