True Seeing spell allows, among other things, this:
and can see into the Ethereal Plane, all out to a range of 120 feet.
Assume that True Seeing doesn't allow one to directly see through the Fog Cloud fog, which is actual conjured fog and not an illusion, and creates a heavily obscured area. I believe this is a common ruling, not a contested topic, and anyway this whole question becomes moot otherwise.
But, Fog Cloud doesn't say it extends to the Ethereal Plane. So, considering above, what does the creature under True Seeing effect see in a fog cloud? Most importantly, does this give any advantage, be it for fighting or moving or anything, over other creatures in the Fog Cloud?
Or, is it simply so, that what ever may be seen on the Ethereal Plane does not reflect what is going on in the Material Plane? Or the fog in fact does heavily obscure the Ethereal Plane, too? Or what?
I suppose related matter is, how are permanent obstacles like walls seen with True Seeing on the Ethereal Plane. My understanding is that they exist as "ethereal material" there, but I am not sure. Fog Cloud creates a temporary spell effect on the Material Plane though, so possibly different.
If this question can't be answered within 5e material, drawing from earlier edition rules or lore is ok.
Best Answer
The way I interpret true seeing is that if you can see an area, then you can see any ethereal creatures or objects in that area (among other things). If you're in a fog cloud, or wearing a blindfold, or have a bag over your head, you can't see at all, so your true seeing is functionally blocked as well.
As far as walls go, we're told that material objects seen from the ethereal side are "muted and indistinct", "shadowy", and that they don't "hamper movement", but nothing says you can see through walls simply by being on the ethereal plane. I see no reason truesight would be different.
The DMG doesn't go into a ton of detail about other planes, but one of my favorite 3rd Edition resources was the Manual of the Planes, which was very specific about this:
That doesn't exactly address the question you're asking, but I think it's clear that the intent is that your field of vision doesn't change when you're seeing into the other side, only what you can see within that area.