Line a 3-1/2- or 4-quart slow cooker with a disposable slow cooker liner. Place bread cubes in prepared slow cooker.
In a large mixing bowl whisk together milk, sugar, eggs, vanilla, and salt. Pour over bread cubes in cooker. Press bread lightly with the back of a large spoon to moisten bread completely. Cover and chill in the refrigerator for 4 to 24 hours.
Remove from refrigerator. Cover and cook on low-heat setting for 7 to 8 hours or until a knife inserted in the center comes out clean. Turn off cooker. Let stand for 30 minutes. Carefully lift liner from cooker, using a plate to transfer French toast to a cutting board. If desired, slice French toast to serve or spoon it into serving dishes. Top with ice cream topping.