Prepare crockpot by spraying well with cooking spray or use a crockpot liner.
Peel, core, and slice all of the apples and place them in the crockpot.
In a separate bowl, combine the sugars, cinnamon, nutmeg, and salt. Pour the mixture over the apples and stir well, making sure to coat all of the apples.
Set the crockpot on low for 10 hours. After the 10 hours, mash any chunks of apple that remain. Allow to cook for an additional two hours.
Remove the lid and use an immersion blender to blend the apple butter until it's smooth. If you prefer, you can let the apple butter cook a little longer with the lid slightly ajar to let it thicken up some more.
Once the apple butter is finished, transfer to containers and allow to cool completely, then refrigerate.