I just bought all the upgrades for it and I was wanting to stash some of items there since I don't have a house and don't want to live in the cities since I'm a vampire...well except for Riften but I'm ran into a glitch so I can't buy it.

I haven't tried it myself , but I assume it would be unless it's glitched . You are the DB leader after all , and the bedroom is yours .
I did notice that the mannequin likes to move around the room though , once it was standing on the floor 3 feet to the right of it's pedestal , and another time it was in the middle of the room in front of the steps up to the bed area .

Yep it's safe to store stuff there

avoid barrels, chests and wardrobes should be safe.

All containers in there are safe. Tested them with 31 days of waiting at Breezehome.

I believe the chests are safe, anything else I would be cautious about putting anything important in there.

All safe. Falkreath Sanctuary is safe up to the point where

You DISLIKED the DB questline???????? I thought it was brilliant and when I first started it and came to the main quest in the DB, I was like YOU WANT ME TO DO WHAT? AWESOME! hahahahaha

But yea, as other's said, containers in your bed room at Dawnstar Sanctuary are save!

yeah I would have been a little upset if I have been using Falkreath for anything, Thats why I just svck it up and buy a house. My first CK mod is going to be a Campsite along the river with a player safe chest!

Cool. I noticed there's a camp site right outside Riverwood, but it only builds up gradually, not sure why it isn't there from the beginning. Maybe add a chest to that one. It has 3 bedrolls and a cooking spit.

I would like a cool cave though, with safe storage

Each to their own. But yeah, it really didn't do it for me. Preferred Oblivion's DB by a long shot.
For a safe container I just bought Breezehome. Pretty cheap, requires next to no effort, right next to Whiterun's entrance. A container for life!

Is it just me or are pretty much all chests safe to store items in, well, except for maybe the big loot chests? I've tested various chests across the game by putting random stuff in them and leaving them, then checking back several in game days later even after they're environment has refreshed and everything is still where I left it.

Hell, I found a treasure chest out on top of a snow mountain and stored an iron ingot in there almost when I first started the game. Randomly stumbled upon it yesterday and the iron ingot was still in there (I'm level 52 atm).

Yeah, and as others have said, I'd avoid using barrels, though. I'm pretty fond of that sanctuary. I really should've waited to blow that free 20 grand until I got the 100k achievement, but oh well, hindsight and all that jazz. The torture room is pretty cool, though, getting treasure locations.