Both faction forts NEVER respawn. Use Harkon's room until you get your own after finishing the questline.
Some stuff does! At least in the Dawnguard fort, I don't know about the vampire's fort. But in the Dawnguard one there's a chest and some barrels that respawn. There's a chest upstairs in the room with the double bed-- every time I enter the fort I can collect gold and gems from it.