Actually, antibiotics would do nothing, as this is not a bacterial disease.
Actually antibiotics would be very effective in treating any infected sores on the skin. Supportive care of the symptoms will help make the dog more comfortable. Robinh should inquire about Baytril to clear up the skin infections, as it has a very low probability of causing GI tract problems. Now back to the original cause...

If you are noticing fluid retention, increase thirst and urination then those are some classic signs of Cushings. If your dog has been on high doses of Prednisone for long periods of time then as stated before that will induce Cushings. Prednisone should only be used for short periods of time to get a dog over the worst of a severe skin problem. Long term treatment should be with antihistamines.