Combine chili sauce and grape jelly in a slow cooker on Medium heat. Preheat oven to 350 degrees F (175 degrees C). Mix ground beef, bread crumbs, onion, eggs, salt, and black pepper together in a bowl; form into golf ball-sized balls. Arrange meatballs on an ungreased baking sheet. Bake in the preheated oven until browned and cooked through, about 10 minutes. Transfer cooked meatballs to the slow cooker and simmer for 20 minutes.