NOTE: It's very important that the milk NOT be added until the very end of cooking as stated below. For best results, in a small mixing bowl combine 1/2 cup liquid from the slow cooker to the milk before adding it back to the slow cooker.
In a 2- or 3-quart slow cooker, combine the first five ingredients. Cover and cook on low 6-8 hours or on high 3-4 hours, until the chicken sausage is cooked through and the potatoes are tender.
Just before turning off the slow cooker, add the milk and spinach and allow to cook just until spinach is wilted and soup heated through, about 5-10 minutes.