Mix the ground beef, breadcrumbs, egg, onion, salt, and black pepper in a large bowl until evenly combined. Shape the mixture into a loaf that fits inside your slow cooker.
Stir together the ketchup, brown sugar, and apple cider vinegar in a small bowl to make the sauce. Place the meatloaf in the slow cooker and pour the sauce over the top.
Cover and cook on low for 6 hours until the internal temperature reaches 160 degrees. While the meatloaf cooks, combine the shredded cabbage, shredded carrots, mayonnaise, sugar, and white vinegar in a medium bowl. Toss until the vegetables are coated and chill until serving.
Slice the meatloaf and serve with the creamy coleslaw on the side.