Heat olive oil in a skillet over medium heat. Add Italian sausage and onion; cook and stir until sausage is browned, about 10 minutes. Transfer sausage and onions to a slow cooker.

Add ground beef to the same skillet over medium heat; season with Italian seasoning, 1 teaspoon of garlic powder, and marjoram. Cook and stir until beef is browned and crumbly, about 10 minutes. Transfer ground beef into the slow cooker.

Stir in tomato sauce, diced tomatoes, stewed tomatoes, tomato paste, 2 teaspoons garlic powder, oregano, thyme, and basil. Cover and cook on Low for 8 hours. Stir in sugar 15 minutes before serving.