Preheat the oven to 350°F. Season a 13x9-inch baking dish with nonstick cooking spray.
Break the spaghetti into thirds. Then cook the pasta according to the package directions. Drain and set aside. Do not rinse.
In a large pot, combine the 2 cans of condensed chicken soup, Velveeta cheese, Ro*tel with liquid (reserve a 1/4 cup to top casserole), cream cheese, milk, Ranch Seasoning and chili powder. Heat over low heat, stirring constantly, until the cheeses have melted.
Add the cooked spaghetti and chicken. Stir to mix thoroughly. Pour mixture into the prepared baking dish and sprinkle the top with the shredded cheddar and Jack cheeses and reserved Ro*Tel.
Bake for 30 minutes or until heated through and the cheese has melted.
If not using rotisserie chicken that has been cooked: Place the chicken breasts in a large pot, cover with water. Cook uncovered over medium heat until cooked through, about 10 minutes. Don't boil the chicken. Allow to cool, then chop into bite size pieces or shred using two forks.
I found using the 16 ounce block of Velveeta too rich (which is the typical amount in most versions of this), but feel free if you like.