Stir both chocolates and cream in slow cooker.

Cover; cook on low 1 hour, stirring every 20 minutes until chocolate fully melts and smooths.

Stir in vanilla until combined.

Spread marshmallows evenly over chocolate.

Increase heat to HIGH; cook uncovered 15 to 20 minutes until marshmallows soften and begin melting.

Serve immediately with graham crackers, strawberries, and banana slices for dipping.