Place a large deep skillet over medium-high heat, add the ground beef, sausage, onion, and bell pepper. Cook for 10 minutes, stirring often and breaking up the meat, until the beef and sausage are no longer pink. Drain any excess grease.
Transfer the beef mixture to a slow cooker. Stir in the pasta sauce, water, vinegar, mustard, Worcestershire, garlic, Italian seasoning, and crushed red pepper (if desired).
Cover and cook on LOW for 4 to 5 hours.
Portion the sloppy joes onto toasted hamburger buns. Top with 3 slices of pepperoni and a slice of mozzarella cheese before putting the top bun in place. Serve warm.