I bought a wireless, mag mount, battery powered camera, mainly for hooking up my trailer. I've only used it twice but it worked well. I can put it on the back of my van to hookup, or put it on the trailer for tight maneuvering.
A lot depends on if you:
Want to move the camera around.
Want a camera just for hooking up (which I did) or every day use in parking lots (needs weather proofing, permanent mount)
Can easily wire (I've a custom Van for 1 vehicle, wiring would be a pain)
Are a skinflint, like me.
I just plug the vid unit into the cig lighter, drop the vid unit on the doghouse, stick the camera on the back of the MaxiVan, back up, hook up, unmount the camera.
I don't think the mag mount would hold while driving down the road.
Since you want one for your trailer you might actually need two, one for either side.