Skinny German Shepard loving milk

My 60 lb German Shepard recently began to loose weight at a rapid pace. I am reacting by beefing up her diet. I was told at 65 lbs to stop feeding her home cooked food because friends thought she should weigh more. That was 3 months ago. She is on Purina Beyond wet food and Nutriese dry food Rachael Ray.

She looks horrible. These store bought foods seem to be harming my baby.
I fed her before real chicken with peas and carrots.
Canned fish (salmon or Mackerel) and small amt of sweet potatoes.
Dry pedigree (as supplement)
Ground Beef and potatoes)

She was on the slim side at 65lbs but healthy and beautiful.
She loves milk and has no side effects from drinking the milk. So my question is :

It's been said milk makes dogs fat but what about a skinny dog needing some fat.? And what do you think about her decline in health after putting her on store bought food.
Picture: Before loosing weight and today