In a small bowl, combine the salsa, Rotel, and taco seasoning. Stir until well combined. In the slow cooker, place the chicken in a single layer and top with the diced onion. Next, gently pour the tomato mixture over the chicken without washing the onions off. Place the cover on the slow cooker, set to low, and cook for 7-8 hours or high 3-4 hours. Once the cooking time is up you can either remove the chicken from the pot and shred it or shred the chicken with 2 forks in the slow cooker. After the chicken has been shredded, give it a quick stir to combine the tomatoes and juices. Serve in your favorite taco shells or tortillas. You can also use this to make nachos, quesadillas, or burrito bowls.