Unity's built-in fog has no affect on the skybox. In the past, for better blending, I would paint the fog into the images using Photoshop. Now, I've created this custom skybox shader to make the process much easier! This brief video shows me adjusting the settings. I'm happy to share this with the game dev community. See resources below...
----------- RESOURCES ----------
Skybox Fog - https://virtueone.com/games/news-view...
Made with Unity - https://unity3d.com/