Cut bacon into 1" pieces. Cook bacon in a large skillet over medium heat until crisp; remove bacon and drain on paper towels, reserving drippings in skillet. (Cover and chill bacon until gumbo is done.) Whisk flour into drippings, and cook over medium heat, whisking constantly, until roux is caramel-colored (about 15 minutes). Add onion and next 3 ingredients; cook until vegetables are crisp-tender, stirring constantly.
Transfer vegetable mixture to a 4-quart electric slow cooker. Stir in okra and next 7 ingredients. Cover and cook on LOW 6 hours. Add shrimp, oysters, and crabmeat; cover and cook on HIGH 1 hour or until shrimp turn pink and edges of oysters begin to curl. Discard bay leaves.
To serve, ladle gumbo over rice into individual bowls. (Microwave bacon 1 minute to reheat.) Sprinkle each serving with bacon. Serve with filé powder, if desired.