I have a wired system, that is on, any time I have it plugged into power outlet in the cab. Purchased individual pieces on eBay.
Here is a similar monitor. It clips onto my rearview mirror. Got parts from Radio Shack, to make a power plug.
Here is a similar camera.
Here is a similar video / power cable. I wired truck and trailer, individually.
I also, have one camera mounted on a piece of aluminum that slides (for hitching up) under the license plate on the truck.
When I hitch up, I plug in (in truck bed) the hitching camera. After hitched up, I unplug hitching camera, put it back in storage compartment, and plug in camera on back of the trailer.