Stir marshmallows, cranberries, and sugar together in a saucepan over low heat; cook and stir until the marshmallows melt completely, about 15 minutes. Transfer marshmallow mixture to a bowl and refrigerate 4 hours to overnight.
Stir apples, grapes, walnuts, and salt into the marshmallow mixture.
Beat whipping cream in a chilled bowl using an electric mixer until stiff peaks form; fold into the marshmallow mixture.
