Preheat a smoker to 250°F. In a small bowl, stir together the softened cream cheese and 1/2 cup of the mozzarella until combined.

In a large bowl, combine the Italian sausage, ground beef, egg, breadcrumbs, milk, garlic powder, and black pepper. Mix gently until evenly combined, taking care not to pack the meat too tightly.

Lay a large piece of parchment paper on the counter. Shape the meat mixture into a rectangle about 1 inch thick. Spread the cream cheese mixture down the center, leaving a border around the edges. Spoon 1/4 cup of marinara over the cheese, then sprinkle with the remaining 1/2 cup of mozzarella.

Use the parchment to help fold the long sides of the meat over the filling. Pinch the seams and ends closed, then shape the loaf so the filling is fully enclosed. Transfer it seam-side down to a smoker-safe pan or rack.

Smoke until the center of the meatloaf reaches 160°F on an instant-read thermometer, about 2 to 3 hours depending on its shape and the smoker. Spoon the remaining 1/4 cup of marinara over the top during the final 20 minutes.

Remove the meatloaf from the smoker and rest it for 10 minutes. Sprinkle with the chopped parsley, then slice carefully and serve.