Place potatoes in the bottom of a 5-quart slow cooker.
In a large bowl, combine ground beef, ground pork, shake and bake, onion soup mix, sliced jarred mushrooms, and egg. Set aside.
In a small bowl, stir together 1/2 cup Cheddar soup and evaporated milk. Add to meat mixture. Using a wooden spoon or clean hands mix thoroughly and form into a loaf. Place in slow cooker on top of the potatoes. Make sure meatloaf does not touch the sides of the slow cooker.
Stir together remaining cheddar soup with cream of mushroom soup. Pour over meatloaf. Top with onions and sliced fresh mushrooms.
Cover and cook on HIGH setting for 1 hour. Reduce the temperature to LOW setting and cook for an additional 5 to 6 hours.