Royal Canin does have corn, see if you can find Old Mother Hubbard Wellness or another without any corn, but remember to switch over slowly by adding a little more of the new each day to prevent digestive upset. Also adding a supplement with Omega 3 and 6 fatty acids also does a lot to help skin irritations (Missing Link, Dermcapps, etc.)
It very well could be the food, but I also think it's still because the fleas are biting, at least add the Avon Skin So Soft to your flea regimen and as soon as it's cold enough for the fleas to die off discontinue the Frontline and see if that helps. Many dogs have allergic reactions to Frontline, Advantage, etc. after all you are putting an insecticide in or on your dog.
http://www.oldmotherhubbard.com/dogs..._overview.html