It took me forever to figure out how to make my sleepers work in the prefabs I was making, and I couldn't find a post about it on here so I figured I would make a quick post and add a short video tutorial for those that want to see it done rather than just follow directions. These instructions assume you know how to make a prefab, capture it, and save it to the prefab file.
There are 5 lines of code needed to be added to the prefabs .xml file. Just copy and paste this into the .xml of whatever prefab you are creating:
<property name="SleeperVolumeSize" value="3, 2, 3" />
<property name="SleeperVolumeStart" value="3, 1, 3" />
<property name="SleeperVolumeGroup" value="S_-_Group_Zom_Soldier" />
<property...