Lightly grease a 6-quart slow cooker.
Add 1 pound large elbow pasta to the slow cooker in an even layer.
Scatter 4 tablespoons butter (cut into slivers) and 4 ounces cream cheese (cut into pieces) evenly over the pasta.
Sprinkle 1 teaspoon salt, 1/4 teaspoon black pepper, 1/4 teaspoon cayenne pepper, and 1/4 teaspoon paprika over the top.
Pour in the 12-ounce can evaporated milk, 1 1/2 cups whole milk, and 1/2 cup heavy cream. Stir gently to combine and press any pasta that is above the liquid so it is submerged.
Cover and cook on LOW for 1 to 2 hours. Check after 1 hour. During the cook, open and stir once more roughly halfway through the total cooking time to prevent sticking.
When the pasta is al dente (still slightly chewy but mostly cooked), add 2 1/2 cups freshly shredded sharp cheddar cheese and 1 cup freshly shredded pepper jack cheese. Stir until the cheeses are incorporated.
Cover and cook on LOW for 10 more minutes to melt the cheeses and finish the sauce.
Stir again to make the mac and cheese smooth, then serve hot.