Bring a large pot of salted water to a boil. Once boiling, cook the pasta for 2 minutes less than al dente, according to the package instructions.
Spray a 4 or 6 quart slow cooker with spray oil, then add in the cooked macaroni. Add the butter, and stir until the butter has melted.
Mix in the half and half, evaporated milk, cheddar, gruyere, Velveeta, onion powder, salt, dry mustard powder, black pepper, and paprika.
Cover and set the slow cooker to LOW heat. Cook for 1 1/2 - 2 hours. Stir approximately every 30 minutes, and make sure to stir as quickly as possible and close the lid quickly so that as much heat as possible stays in the slow cooker.
Once the mac and cheese is finished cooking, stir until you have a creamy and smooth mac and cheese sauce that is fully coating the noodles.