Place chicken in the slow cooker.
Sprinkle taco seasoning, corn, black beans, and onions over chicken.
Pour salsa and canned tomatoes on top.
Cook on high for 4 hours or on low for 6-8 hours.
Just before serving, use two forks to shred the chicken.
Add cilantro and stir to combine ingredients.
Serve over brown rice with optional toppings as desired.