Use a 5–6 quart slow cooker.
Grease the insert with butter or nonstick spray to prevent sticking.
Soften the cream cheese so it melts smoothly.
Add milk, evaporated milk, cream cheese, butter, and all seasonings to the crockpot. Whisk until mostly smooth. It’s fine if tiny cream cheese bits remain.
Stir in the dry pasta and most of the cheese: add 2 cups cheddar, 1 cup mozzarella/Jack, and 1/4 cup Parmesan. Reserve the remaining cheese for finishing.
Cover and cook on Low for 1 hour. After 30 minutes, lift the lid and stir well, scraping the bottom and corners to prevent sticking.
At 60 minutes, stir again. Check pasta tenderness. If still firm, continue cooking on Low for 20–40 minutes, stirring every 15 minutes.
When pasta turns tender and sauce looks creamy but slightly loose, turn off heat. Stir in remaining cheddar and Parmesan until melted. The sauce will thicken as it rests.
Taste and adjust seasoning. Add a splash of milk if it seems too thick or a pinch of salt if it needs pop.
Switch to Warm for serving and stir every 15–20 minutes to keep the sauce smooth.
