I had a similar fog problem for my Underwater Project Im working on. I ended up using several overlaping Spheres flipped and centered on the cammera, each sphere is alpha shaded to about 10% opacity then either textured or colored form the desired effect. Fist sphere is at a distance of 5 then the next 9 are at 10, 15, 20, etc. What happens with this is that the objects get less vissable the further they go till after sphere 8 or so they are faint shadows.
For a moving smoke like fog effect just stick on a animated smoke texture to the first few spheres.
For a moving smoke like fog effect just stick on a animated smoke texture to the first few spheres.