I have beat the main storyline of Dawnguard on the vampire side. After killing Harkon, I went into the room with the bed/enchanting table/alchemy table to try to sleep, however it gave me the error "Cannot sleep in a bed that is owned."

This is obviously a room made for player!

Nope I believe that is either Serana's room or the moth priest's room, you get lord Harkon's old room.

Yup I thought it was mine too, we get Harkon's old room which is not as good.