Fill a large pot with lightly salted water and bring to a rolling boil; stir in tortellini and return to a boil. Cook uncovered, stirring occasionally, until tortellini float to the top and filling is hot, about 10 minutes. Drain and set aside. Preheat the oven to 375 degrees F (190 degrees C). Heat a large skillet over medium-high heat. Cook and stir Italian sausage in the hot skillet until browned and crumbly, 5 to 7 minutes, breaking into bite-sized pieces as it cooks. Drain and discard grease, but do not rinse or clean the skillet. Place cooked sausage into a 9x13-inch baking dish. Heat the same skillet over medium heat. Cook and stir onion until slightly browned and tender, 5 to 7 minutes. Add bell peppers, zucchini, and squash; reduce heat to low, cover, and cook until vegetables are crisp-tender, 3 to 5 minutes. Pour in diced tomatoes, Neufchatel cheese, Italian seasoning, salt, and pepper. Heat until combined and smooth, about 5 minutes. Pour cooked tortellini into the baking dish with sausage. Add vegetable mixture and stir gently to combine. Drizzle with marinara sauce; top with shredded Parmesan cheese. Bake in the preheated oven until cheese is golden, 25 to 30 minutes. Let cool 5 minutes before serving.