on fords you can disconnect the tailgate camera and plug an extension cord in its place and tun it to the back of the trailer to a compatable camera. Obviously not wireless.
I think the LockPick system which lets yiu add cameras using the trucks monitor is a really good plan.
We didn't do the LockPick because DW didn't want anybody taking the dash apart to connect it to the back of the monitor.
Instead we went with a TadiBrothers separate monitor and a wireless camera to keep an eye on the horses in the horse trailer and it works so well we are going to add one on the camper also for review viewing not just backing up.
Jon