Dermatitis due to allergic reaction

over the last few weeks my dog has had lots of scabs appearing on the side of her face its very noticable as she is mainly white. the vet thinks this could be an allergic reaction to something giving her contact dermatitis she has been given some spot on advocate and i will need to bathe her in some special shampoo for 4 weeks then take her back to the vet. i was told a reason this may have happen is because of the age usually happens between 2 and 3 with white dogs.shes not having much luck in 2 years she has suffered intemittently with conjunctavitis has had a hysteocondroma removed now this bless was wondering has any one elses dog sufferered with allergies and whats the best things food wise to give her to prevent her suffering with these scabs any more at he mo she is fed raw as she s not to keen on kibble thanks to any one who posts a reply.