Boil the macaroni in water with 1 teaspoon salt for 3 minutes less than the package directs and drain it well.
Coat a 6-quart crockpot with the butter and add the pasta, evaporated milk, whole milk, mustard, pepper and remaining salt.
Cover and cook on low for 45 minutes, then stir from the edges toward the center.
Add the cheddar and American cheese in 3 additions, stirring each addition into the hot liquid.
Cover and cook on low for 45 to 60 minutes more until the pasta is tender and the sauce is smooth, then switch to warm for no longer than 1 hour.