Funny, I was about to post a similar question.
It's a long and expensive road, but what are you gonna do?
My australian cattle dog has been diagnosed for about 2 years now. The vet had him on Hill's W/D dog food (which is full of corn and HORRIBLE). I pulled him off of that and put him on Wellness core. It's a bit expensive, but well worth it. From being on this food, his insulin levels have stabilized and he began losing weight.
Sawyer will be 6 years old this may and upon diagnosis, he weighed 67 pounds. He was a little chunky, but had been on Beneful Healthy Weight Formula all his life (he's an inside dog). I personally believe that the high sugar content in this food is what led to his condition. He has since dropped 12 pounds and I feel like I got my dog back. He's very energetic and playful.
My dog is on 12 units of Vetsulin twice a day. I feed him right before each shot. We have had absolutely zero complications, except for owner error. (If we miss a dosage, there's usually an accident in the house)
I will say this, my vet was amazed at the difference in him after I put him on Wellness Core. There are many other foods out there that are similar and very good. I had decided to go grain free and liked the ingredients in it.
This site is a great one to look into food content.