Heat the oil in a skillet on the stove top. Add the diced peppers and onions and cook until tender. Stir in the chicken and fajita seasoning. Set aside.In a large sauce pan, combine the cubed Velveeta and the entire can of Ro*Tel tomatoes. Heat on medium heat until creamy. Stir in the fajita chicken mixture. Serve warm with chips, crackers, or fresh vegetables.