Cook noodles according to package directions. Meanwhile, in a large skillet, cook beef and onion over medium heat until meat is no longer pink, breaking it into crumbles; drain. Stir in the spaghetti sauce, mushrooms and Italian seasoning. Drain noodles.
In a 5-qt. slow cooker coated with cooking spray, spread a third of the meat sauce. Cover with a third each of the noodles and pepperoni. Sprinkle with a third of the cheeses. Repeat layers twice. Press down to compact.
Cover and cook on low for 2-3 hours or until heated through and cheese is melted.