Lightly grease the inside of your slow cooker with cooking spray or a little butter.
Place sweet potatoes in a colander for 10-15 minutes to let them drain. Pat dry with paper towels if they seem especially wet.
In a large mixing bowl, add the drained sweet potatoes and mash with a potato masher until smooth (a few small chunks are okay). Add melted butter, sugars, milk, eggs, orange juice, vanilla, and cinnamon. Stir until everything is smooth and well combined. Pour into the greased crock pot and spread evenly.
In a small bowl, mix together pecans, brown sugar, flour, and melted butter until crumbly. Sprinkle evenly over the top of the sweet potato mixture.
Cover and cook on LOW for 4 hours, or until the edges are set and the center is hot and slightly puffy. Cook with the lid off for the last 15-20 minutes to let it set up more.
Scoop and serve warm.