Agree with the others about testing for EPI.
I had a dog like this. She never had an issue with food and then all of a sudden she would practically eat the metal bowl! She was eating about 10 cups of food per day and went down to 50 pounds!
Your dog is probably not assimilating her food properly thus the thinness. A good quality digestive enzyme may help in the mean time before testing. This one is sometimes found in local Doggie Boutiques, or you can purchase from the company. The Wholistic Pet
Hope this helps!