Might want to try Satin Balls . Great for fattening up. If you look on google, you will find recipes that are similar but varied, incase you don't have the right ingredients on hand.

Mucous in the stool can be causes by intestinal worms. Definatly get her checked for that, and keep the other dogs away from her stools or anything her stools might have touched, including her bum. It would be a good idea to wash their paws after they come in from the outside.

Intestinal Worms