Pour 1 cup of spaghetti meat sauce into a 4- to 6-quart slow cooker. Place half of the noodles and half of the ricotta mixture on top of the sauce. Cover with 2 cups meat sauce. Top with remaining noodles (if the noodles don't fit exactly, break them to fit), cheese mixture and meat sauce.
Cook on LOW for 4-5 hours or until noodles are soft, being careful not to overcook. (Don't try to speed up the process by cooking on high.) Sprinkle 1/2 cup of mozzarella and remaining Parmesan on top. Cover with the lid to melt the cheese and allow to sit 10 minutes before serving.