Place beef in a 4-quart slow cooker and set aside.
Stir together tomatoes and fajita seasoning mix in a bowl; pour over beef. Cover and cook on high setting one hour; reduce heat to low setting and cook 8 hours.
Add onion and green peppers; cover and cook on low setting one more hour. Remove beef and vegetables from slow cooker; shred beef with 2 forks. Serve beef mixture on warmed tortillas with desired garnishes.