Rub soup mix all over pork roast. Place roast in a slow cooker and pour cranberry sauce over top. Cover and cook on Low until pork is slightly pink in the center, about 8 to 10 hours. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C). Transfer roast to a platter, reserving juices in the slow cooker.
Melt butter in a saucepan over low heat. Whisk in flour to form a paste. Pour in reserved juices from the slow cooker; cook, whisking constantly, until gravy lightens in color and thickens, about 5 minutes. Season with salt and pepper. Serve alongside roast.