I am sorry to hear about Ginger's recent troubles. The high calcium level makes me worried that she has a condition called caliconsis cutis. This where calcium deposits in the skin causing sores. The best treatment of to reduce the calcium levels and prevent calcium from being deposited in the skin. There are several potential causes of high calcium including renal disease (secondary hyperparathyroidism), primary hyperparathyroidism, fungal disease, and toxicity. Addison's disease is also a reported cause but not likely to be present as long as the dog is on steroids.
It sound like your dog needs to be worked up thoroughly for each of theses causes. Some can be ruled out with routine blood work while others require special testing. Your vet can send out a hypercalcamia panel to test for primary hyperparathyroidism as well as cancer related hypercalcemia. Other testing that should be pursued if you would like to get a more definitive answer may include an abdominal ultrasound (plus or minus fine needle aspirate of any abnormal organs), chest x-rays and a skin biopsy or FNA of the lymph nodes. Another thing that your dog should be evaluated for is the presence of an anal sac mass as this would suggest cancer of the anal sac gland and removal would be recommended.
How far you want to go to diagnose your dog's issue is up to you. Prednisone can cause calcium wasting but can also cause immune suppression which can predispose patients to infections.
If you would like to pursue additional testing but your vet is not able to to do all of these things you may also ask about a referral to the nearest internal medicine specialist. He or she can make the appropriate recommendations based on your dog's specific condition and you can decide together how you want to treat your dog.
Skin cancer does not usually cause hypercalcemia so this would not be my first suspicion. The most common cancers that cause hypercalcemia are lymphoma (T cell is more common than B cell to cause this), anal sac adenocarcinoma and multiple myeloma.
Does this help you answer your questions today?