Lightly grease the inside of a 6-quart slow cooker.
Spread one-third of the marinara in the bottom. Add half the frozen ravioli in an even layer.
Scatter half the cream cheese in small pieces over the ravioli. Add another third of the sauce and 1 cup mozzarella.
Layer the remaining ravioli, cream cheese, and sauce. Cover and cook on LOW for 3 to 4 hours, checking at 3 hours, until the ravioli are tender and hot throughout.
Scatter the remaining 1 cup mozzarella over the top. Cover for 10 to 15 minutes more, until melted. Serve the layers as they are, or stir gently for a creamier sauce.