Boil the macaroni noodles according to the package instructions until al dente. Drain and set aside.
Mix the cooked noodles, milk, evaporated milk, butter, salt, onion powder, dry mustard, 2 cups of the cheddar cheese, and the Velveeta in the slow cooker.
Cover and cook on low for 4 to 5 hours, stirring every two hours.
Sprinkle the remaining cheese over the top, cover, and allow the cheese to melt just before serving.
Serve immediately for a creamier texture or allow the macaroni to rest for 15 minutes to thicken.
