In a large skillet over medium high heat, cook ground beef, onion, and garlic until meat is no longer pink. Drain any excess fat.
Stir in marinara sauce, drained tomatoes, and Italian seasoning. Simmer for 5-7 minutes. Season with salt and pepper.
Spray a 6 quart slow cooker insert with cooking spray. Spread 1 cup of the meat sauce on the bottom.
Add half of the frozen ravioli in a single layer. Top with 2 cups of sauce, then 1 ¼ cups of mozzarella cheese.
Repeat layers: remaining ravioli, remaining sauce, and remaining 1 ¼ cups mozzarella. Sprinkle Parmesan cheese on top.
Place a double layer of paper towels over the top of the slow cooker (under the lid) to absorb moisture.
Cover and cook on LOW for 3-4 hours, or until ravioli are tender and cheese is melted and bubbly.
Turn off slow cooker, remove lid and paper towels, and let rest for 15-20 minutes before serving.