How much you can expect from $20 wireless transmitter?
All back up cameras, beside back up wire that turns it on automatically when on reverse, also have manual switch that will turn it on when you push it.
Also don't expect too much from them. Small camera can be bought as low as $30 and will give you view of your bumper and trailer, but not much of traffic view.
I have wired camera with big LCD that clamps on rear view mirror.
Don't use it often, but put long wire and I activate it buy pushing plug into the lighter socket.
This way I can move it between TC and camper shell.