Lightly spray the slow cooker with cooking spray. Mix peaches and berries together in the slow cooker; sprinkle with cornstarch and toss to coat.
Stir vanilla into berry mixture; add brown sugar, cinnamon, and nutmeg. Pour cake mix over berry mixture and drizzle with melted butter.
Cook in the slow cooker set to High until bubbling, 3 to 3 1/2 hours.
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