Substituting parchment for foil in slow cooker
My experience would suggest that unless the aluminum foil is used to form some kind of shape such as a seal around the food, parchment paper would be superior.  Aluminum foil is better for forming shapes.  Parchment paper is better for non-stick qualities.  Parchment paper has a temperature limit of around 400 degrees F, whereas aluminum foil will tolerate much higher temperatures (technically it will melt around 1200 degrees F).  Most slow cookers wouldn't be too hot for either one, but they vary, so you may want to check yours to be sure.