In a 5- to 6-quart slow cooker, combine the tomatoes, bell peppers, onion, oregano, cumin, 11/2 teaspoons salt, and 1/4 teaspoon pepper. Nestle the steak among the vegetables. Cook, covered, until the meat is tender and pulls apart easily, on high for 4 to 5 hours or on low for 7 to 8 hours.
Twenty-five minutes before serving, cook the rice according to the package directions.
Using two forks, shred the beef and mix it into the cooking liquid. Serve with the rice and top with the avocado and cilantro.