Royal Canin has been known to make Yorkies very itchy. IMO it's not a very high quality food and I would change. It is very likely you'll see a decrease in her symptoms once you switch.
The best possible diet for your Yorkie is raw. Nature's Variety and Primal both make great pre-packaged bags of medallions.
As far as kibble goes, I would recommend a couple of brands. Brands we've tried and loved include California Natural and Wellness. I LOVE Wellness. Other great brands include Taste of the Wild, ZiwiPeak, Orijen, and Nature's Variety also makes kibble. I prefer all natural, grain-free diets though.
Dog Food Analysis - Reviews of kibble is a great resource on foods. The premium foods are listed as 4-6 star. You can start your research there.
Hope this helps!
