Preheat oven to 350 degrees. Lightly spray a 9x13" pan with non-stick baking spray. Mix cake mix, water, oil, and eggs according to package directions. Pour batter into the prepared pan and bake for 30-35 minutes (or according to package directions). The cake is done when a toothpick inserted comes out clean and crumb-free.
While cake is still hot, use the back of a wooden spoon to poke holes in the cake about 1" apart. Warm the strawberry jam in the microwave (about 30-60 seconds) and stir. Pour the jam into the holes of the cake. Let the cake cool completely.
Mix the whipped topping: With a hand or stand mixer, mix the cream cheese and powdered sugar. Add the milk and cream together to a smooth consistency. Add the full container of whipped topping and mix just until combined. Top the cooled cake with the whipped topping and spread evenly over the cake. Add the sliced strawberries to the top of the cake. Refrigerate until ready to serve.