So, in attachment, a scene file, whose contain the setup I used. The most important is the scale of the scene, intensity of emitters and the scale of the scattering node, that determine the density of fog. To much density, the scene turn to black, cause to much absorbtion by the volume. You need to change scale of scattering by scene. if you unplugged the mix node, from scattering you will have a uniform fog. Be carefull, to bounding box your volume fog to never cut it with the camera plane, the camera need to be always out of the box. One setup is to use, a box that match your frustrum camera, more easier setup.
I put also a screenshot from my maya setup, and result from the octane scene.