Preheat the oven to 350 degrees. In a large bowl, mix together the shredded chicken, 1 cup of cheese, cream of chicken soup, milk, sour cream, Ro-Tel tomatoes, and taco seasoning mix. Stir well.
Grease a 2-quart casserole dish. Layer half of the crushed Doritos across the bottom of the dish. Top with half of the chicken mixture. Add another layer of crushed Doritos, and then add another layer of the chicken mixture. Top with remaining 1 cup cheese.
Cover and bake for 30-35 minutes, until bubbling hot. Remove from the oven and top with chopped lettuce and diced tomato or more Doritos, if you wish.