Sorry, this is going to be long to explain.
My Vizsla Blaze had a tick disease Ehrlichiosis. Terrible cruel disease. One of the things it did was mimic Masticatory Muscle Myositis with her immune system attacking the bones and muscles of her head including her jaw. Her jaw pain was so intense she could not eat or drink. She initially was diagnosed with both the tick disease and MMM which presented a problem. The tick disease called for treatment with doxycycline and the MMM with Prednisone. The two together presents complications not good for the dog. My research found me on a tick disease listserve with dozens of people having dogs going through or gone through similar problems as well as self educated experts and veterinarians who had a special interest in tick diseases. What I learned was that it was not unheard of for ehrlichia to present symptoms that mimic an autoimmune MMM issue. I had a tough decision on my hands and chose to not do the prednisone and to do both a higher dose of doxy and for a longer duration than what the merk manual called for. I made the right decision as Blaze is still with me at almost 12 years old. By luck as it just as easily could have been the wrong call and killed her.
Now that was my story and I am not saying your dog has a tick disease. But it might not hurt to have your vet to run a titer test for tick-borne diseases if not already.
My dog presented as a dog with MMM so caring for her was no different. Her head muscles you could tell where visibly wasting by comparing previous pictures of her. She could barely open her jaw and to touch her jaw was painful. It got to the point I was hydrating her subcutaneously twice a day with an IV between her shoulders. She would look like a camel until it was absorbed. We made up a liquid diet we called "Blaze Soup" that we could help her eat.
What I do know is if your dog's MMM was caught early enough and on prednisone at a sufficient dose long enough, she should recover and be back to normal. If there was any muscle wasting if probably will not come back, but for my dog it has not been a problem and no one notices it but my family. Use of her jaw soon was back to normal. Some teeth were slightly loose afterwords from some bone loss from the MMM symptoms. We did have to stop a vet who wanted to pull some teeth while doing a deep teeth cleaning on Blaze explaining to him it was due to a tick disease and not gum disease. She still has all her teeth, has retrieved magnum geese since, and still eats just fine.
Feel free to ask any questions public or pm. Our dog's situations are different, but alot of what you are going through may be the same. http://texashuntingforum.com/forum/ubbthreads.php/ubb/showflat/Number/2998284/