Prep the slow cooker. Lightly grease the crock with nonstick spray or butter.
This helps prevent sticking and makes cleanup easier.
Layer the apples. Add the apple pie filling to the bottom of the crock. Stir in cinnamon, nutmeg, vanilla, and a pinch of salt. Spread into an even layer.
Add the dry cake mix. Sprinkle the cake mix evenly over the apples.
Do not stir. Try to cover every bit of the apple layer.
Drizzle the butter. Pour melted butter evenly over the dry cake mix. Aim to moisten as much surface area as possible.
Dry spots can lead to powdery patches.
Cook low and slow. Cover and cook on High for 2 to 2.5 hours or on Low for 4 hours, until the top is set and the edges are golden and bubbling. Every slow cooker runs differently, so start checking 15–20 minutes early.
Rest before serving. Turn off the heat and let it sit, uncovered, for 10–15 minutes. This helps the topping firm slightly while the apple layer stays saucy.
Serve warm. Spoon into bowls and top with ice cream, whipped cream, a drizzle of caramel, or chopped nuts for crunch.