Combine all of the ingredients (expect the toppings) in the bowl of a slow-cooker. Give it a stir to dissolve the sweetened condensed milk and to evenly distribute the chocolate chips. Cook on low for about 2 hours, stirring every now and then to ensure everything is well combined. Keep warm until ready to serve. Serve with various toppings like whipped cream, marshmallows, sprinkles, cookies, chocolate syrup, candy canes, butterscotch chips, white chocolate chips, and anything else you like! Enjoy!