Peel sweet potatoes and slice 1/4 inch thick. Place in a 6-quart or larger slow cooker. Add orange zest, orange juice, 2 tablespoons each butter and brown sugar, vanilla, cinnamon, 1/2 teaspoon salt, 1/4 teaspoon pepper and nutmeg. Stir to combine. Cover and cook until the sweet potatoes are tender, about 3 hours on High or 61/2 hours on Low. Mash with a potato masher until smooth.