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 name="SleeperIsLootVolume" value="False" />
<property name="SleeperIsBossVolume" value="False" />

Save your .xml and reload the prefab editor.
While in the prefab editor menu. use the 'level tools' tab and select 'Show sleeper volumes'
Use the standard editing keys to select and move the volume to the appropriate sleepers you want covered. (G=move selection, Shift+G= change selection size, left click=select the selection area)
Use 'K' to enter the volume properties and make changes as necessary, selecting a group, changing the spawn rate, etc.
Use the prefab editor menu 'level tools' menu again to select 'CPY SLEEPER VOLUME' if you would like to add additional volumes. This will create a duplicate of the selected volume directly above that volume. You will need to change any/all values in the properties to suit your needs.
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 name="SleeperIsLootVolume" value="False" />
<property name="SleeperIsBossVolume" value="False" />

Save your .xml and reload the prefab editor.
While in the prefab editor menu. use the 'level tools' tab and select 'Show sleeper volumes'
Use the standard editing keys to select and move the volume to the appropriate sleepers you want covered. (G=move selection, Shift+G= change selection size, left click=select the selection area)
Use 'K' to enter the volume properties and make changes as necessary, selecting a group, changing the spawn rate, etc.
Use the prefab editor menu 'level tools' menu again to select 'CPY SLEEPER VOLUME' if you would like to add additional volumes. This will create a duplicate of the selected volume directly above that volume. You will need to change any/all values in the properties to suit your needs.