My corgi has perivulvar dermititis - not responding to treatment. Have tried cortisone cream & tablets, anti fungal cream, weeping wound powder. Any suggestions?

If these topical treatments are not working for her it is time to see the vet and determine if she needs oral antibiotics. While you are there ask about Malecetic Wipes or something similar that has a drying agent to help keep the area clean. Many people use these for skin folds around the nose and around the tightly curled tails of their bulldogs.
Another option is surgery to lessen the amount of skin that becomes affected, personally I would consider this ONLY if properly cleaning and oral antibiotics don't solve the problem, I don't think dogs should be subject to unnecessary anesthesia and surgery.

Have these treatments been initiated by your veterinarian? Has she been on antibiotics? Has a culture been done?