My character lives in Fort Dawnguard now, she now kinda considers it home... Is it safe to store inventory there ? I found some empty chests... Thanks for your replies!

I don't know how up to date this is but Ford Dawnguard does not appear on the list.

http://www.uesp.net/wiki/Category:Skyrim-Places-Safe

You could select a chest, and store some items in it and wait 31 days and see if it respawns.

ok i'll try that out. Thanks!

The few chests in your room in the Dawnguard fort, which is upstairs on the other side of the circle (from the stairs).

I believe those are the only safe places but I might be wrong.