Slow Cooker Harvest Stew

2 cups cubed Yukon Gold potatoes 2 cups cubed, peeled sweet potatoes 2 cups butternut squash, peeled, cubed 1 cup cubed rutabaga 1 cup diced carrots 1 cup sliced celery 1 cup diced onion 2 cloves garlic, minced 1/4 teaspoon fresh sage, chopped 2 cups apple cider 2 cups vegetable stock 1 Granny Smith apple, thinly sliced 1 teaspoon kosher salt 1/2 teaspoon freshly ground black pepper