I think i have the answer to your problem.
I am a vet tech and i have raised several large breed dogs.My suggestion is a frozen dog food called bil-jac you can get it at feed stores and usually pet's mart.It has a high fat and protein content so follow the directions.It should take 2-4 weeks to see visual results.Good Luck!

I'm not sure how well this will work for a dog, but when we need to put weight on the neglected and abused horses, or just the hard keepers we make sure they drink lots and lots of water. If you can't make him drink lots you could add electrolytes to it (its a powder purchased at any feed mill) We also gave them a little extra exercise. That makes them hungry and builds their metabolism as well as their muscle. We add canola oil to their feed (1/4 cup per day for a dog) but that tends to be very high in fat. We recently found a product that is better for the animals right now. It is healthier and it puts on weight faster, we are giving it to all the skinny little race horses. It is called "EQUIJEWEL" It is a rice and bran based product that we feed in the morning which seems to be working great. Good luck with your dog.

get the vet to check for EPI ?
Exocrine pancreatic enzyme deficiency
is common in Sheperds.
it is treatable with pancrex powder, however it is for life

Also, take into consideration if she was severely neglected (same for humans) that her body is trying to make up for lost nutrients internally before it begins putting on weight.