Heat olive oil in a large skillet over medium heat. Add ground beef and bulk sausage. Cook, breaking up meat, until browned and cooked through, about 7-8 minutes. Drain excess fat.
Add chopped onions and garlic to the meat and sauté until onions are softened, about 4 minutes.
Stir in dried basil, dried oregano, ground black pepper, and salt. Cook for 1 more minute to release the aromas.
Transfer the meat mixture to the slow cooker. Add tomato sauce, crushed tomatoes, diced tomatoes, and chopped roasted red peppers. Stir to combine.
Cover and cook on LOW for 3 to 3 1/2 hours or HIGH for 1 1/2 to 2 hours.
About 30 minutes before serving, cook the ziti pasta in salted boiling water until just al dente, then drain and rinse with cold water to stop cooking.
In a large bowl, combine cooked pasta with ricotta cheese and half of the mozzarella cheese.
Stir the pasta mixture into the slow cooker sauce and meat mixture. Mix gently to combine.
Sprinkle remaining mozzarella and Parmesan cheese evenly on top.
Cover and cook on HIGH for another 25-30 minutes until cheese is melted and bubbly.
Let stand for 5 minutes before serving to allow the casserole to set.