You and your dog have been through a lot. I would suggest you try Derma-IonX. It helps to hydrate the cells and cleanses the tissues; it also helps to improve cellular and enzyme function. I would also consider feeding a grain-free diet and gradually giving more raw foods. Digestive enzymes and probiotics are generally helpful, too. The amount of drugs that have been used has changed the intestinal flora. Ask your veterinarian about getting Purina's FortiFlora. I have gotten good results using it. One cannot be really healthy if the microbes inside are not the right kind that work for the body. Also check for parasites, sometimes they are making a difficult situation worse.
Hope you will see some improvements with Derma-IonX quickly, but remember true healing will take some time due to the chronic state of the situation. Hope to hear of your progress.