Boil a large pot of salted water and cook the jumbo pasta shells until al dente, about 9 minutes. Drain and rinse under cold water to stop the cooking process, then set aside.
In a large mixing bowl, combine the shredded chicken, ricotta cheese, half of the grated Parmesan cheese, 1 cup of shredded mozzarella cheese, garlic powder, Italian seasoning, salt, and pepper. Mix well until all ingredients are fully incorporated.
Take each cooked pasta shell and fill it with the chicken filling, about 2 tablespoons per shell. Place the stuffed shells open side up in a greased 9x13 inch baking dish.
Pour the Alfredo sauce evenly over the stuffed shells, ensuring they are well covered. Sprinkle the remaining mozzarella and Parmesan cheeses on top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 20 minutes. Remove the foil and bake for an additional 10 minutes or until the cheese is bubbly and golden brown.
Remove from the oven, let cool for a few minutes, and garnish with fresh chopped parsley before serving.