Yes you are correct that there are no 12 v dc power points, except of the "Zamp" connector on the outside.
I needed a cigarette lighter style socket in the rear of the trailer for a Lenovo 12 volt
PC power supply. It was when the trailer was brand new and my spouse was not yet comfortable with having holes drilled in it.
I happened to have an appropriate wire gauge extension cord with a cigarrette lighter socket on one end and the plug on the other end. I cut off the plug, routed the wire from the back of the trailer through the slot under the back of the divider between the cabin and the water pump area and into the WFCO. Then I attached the red wire to one of the open fuse connections in the WFCO power distribution block, and connected the black wire to the ground connection at the WFCO (just loosened the screw on a #6 white wire, pushed the black wire in and retightened screw.
Closed think up and done... You should choose a fuse rating based on your load and wire size.
You should have a WFCO manual in your packet -- if not online -- shows how its assembled and wired..