I've always wondered this, but why don't any of the maps for Skyrim properly show Fort Dawnguard or Castle Volkihar. Both the in-game map and the wiki have the markers, but neither have been updated to properly show them. Fort Dawnguard is especially badly served, with even its marker being waay off where it should be.

Well, they're both right off at the edges of the map, which is already kind of... distorted and not super reliable. Fort Dawnguard's map marker is probably actually in about the right place, it's just you can't directly get to it, as it's in the 'Dayspring Canyon' (I think??) worldspace, so if you walk towards the FD marker then it seems wrong. But if you go to DC, if I remember rightly, it's a longish walk eastwards to the actual fort. What there isn't, is a map marker for Dayspring Canyon's cave entrance thing.

But if I remember right, FD is north-northwest of the tunnel entrance, which would put it just inside the Skyrim boundary on the map.

I agree with valindril in that my only complaint is no map marker for the access canyon. Both Giants Grove and Forgotten Vale come to mind as locations on the map that are only accessible via another dungeon. There is also at least one Forsworn location that is like that.