So does anyone actually know if either location has safe storage?

I don't mean safe like the 'barrels outside warmaidens' which reset after 30 days..

Actually safe like the purchasable homes.

Bumping to see if anyone knows the answer to this. I haven't come across any obvious "player area" places in either location, despite having completed the Dawnguard MQ with both factions on different characters.

Has anyone tried the chest in Harkon's private room?

I heard about Dawnguard one single room somewhere on the second floor room

Fort Dawngaurd has chests on the first floor, in the hall opposite the training rooms and the dog kennels.

Ive used them from start to finish whilst doing the Dawnguard quests, they dont reset.

This isn't massively helpful though, some containers have a 30 day (game days) reset, like the barrels outside the Warmaidens in Whiterun.

Unless you (or anyone) would be willing to test out the containers by waiting 30+ days then checking, I would do it myself but I'm away from my Xbox for a bit.

All my stuff has been in there for way more than 30 days... they are not crates or barrells.. they are chests..

well i know the chest in dining area respawns. after you finish the dawnguard quests the room opposite the second floor stairs has non respawning storage. all in the room accept the barrels are safe.

As a vampire, I used the two dressers in the basemant (the two beside the coffin you can use, with Vamp armor stacked on them)
Not sure if it's been over 30 days, but they haven't respawned yet.

I assume the chest in Harkon's room is safe after you finish the quest. because the coffin becomes available to you. so I think the room is supposed to be yours.

I know upstairs in Fort Dawnguard there is a room that I"m pretty sure is suppose to be yours (not Isran's but kind of next to it). There is even a weapon rack there. However, when i used it, I could not retrieve my weapons from the rack. Don't know if this is a common bug or not, but i've never had that problem with any of the weapon racks in the normal vanilla game houses. Had to reload because I put Auriel's Bow on it.