Add the semi-sweet chocolate chips, milk chocolate chips, and heavy cream to your slow cooker and stir to combine.
Cover and cook on low for 1 hour, stirring every 20 minutes, until chocolate is completely melted and smooth.
Stir in the vanilla extract until well combined.
Spread the mini marshmallows evenly over the top of the melted chocolate.
Increase heat to high, leave uncovered, and cook for 15-20 minutes until marshmallows are golden brown and toasted on top.
Serve immediately with graham crackers, fresh strawberries, and banana slices for dipping.