I found a letter on the corpse of a dawnguard member by the name of Lynoit. He had been instructed to leave wilderness supply drops in 3 different locations. I've been searching for the one east of the ruins of Volskygge but can't find them, anyone had any luck with this?

Look for strong boxes. I found one such dawnguard person and had three locations as well. Mine where under the dock by the nightingale inn, east of the mill in Morthal and under the great arch outside of Winterhold.