Most apple butter recipes I came across used a mix of brown sugar and granulated sugar and suggested removing the peel. Removing the peel definitely makes the apple butter more luscious and smooth but I opted to keep the skin on. You can also adjust the sugar per your taste as well as sweetness in apples/type of apples you use for the recipe.
Apple cider vinegar or apple juice is added to the slow cooker in order to help apple cook/release juices and it also prevents it from sticking to the bottom.