My Campbell's Dwarf is thin but rejecting food. What should I do?

She is almost 18 months old with no apparent sickness. She has always had poor eyes, as she keeps them closed unless interested. Her fur coat is perfect and so is the undercoat, with only minor balding behind the ears. She seems to be old, as when she walks, she trembles slightly and moves rather slowly in general. She has had three litters, but the third was unintentional. Her cage is clean and the bedding is kiln-baked pine, but phenols would have different effects. Lately, she has rejected most of the food I give her, including peanut butter with white bread. Her water is always accessible and is frequently refilled with PUR-filtered water. I am considering giving her milk if she will take it like long time ago. What should I do to boost her appetite if I can? No disrespectful or poorly thought responses. Spelling and grammar MUST be acceptable.