What vitamin product are you using? Slings can be good if they tolerate being in them. Of course take her out often to sleep or to try and get around and stretch.
Welcome to BYC. I had an older hen who had this problem for 5 weeks last winter. I had to feed her 3 times a day, chicken feed with water, egg, cat food, and tuna.I gave her vitamins with riboflavin (B2) and she finally got better and started walking again. My hen never lost the strength in her...