I tried this recently and found the moisture content wasn't enough to cook the potatoes thoroughly, without adding the milk to the soup. If you don't want to use milk, mix the soup with water. You can even choose to use cream of chicken, cream of onion/potato, or cheddar cheese soup.
I love my mom's scalloped potatoes but I can't stand the shallow dish resulted in cooking in the baking pans, and I was looking for a nice deep dish recipe for my favorite creamy, cheesy potatoes! The crockpot does this wonderfully.
Watch your cooking times. My grandma's old Rival Crockpot takes a bit longer than a newer, non-stick metal one might. A long clean butter knife inserted into the middle, all the way down to the bottom of the dish should come out fairly clean (but probably cheesy!) and not have much resistance when you poke into the potatoes (or when pulling the knife out).