Back-up camera system.

Never seen one.
You might be better off getting one of the blindside cameras that goes on the passenger side, don't know what they're called but USX used to have them on most or all (?) of their trucks.
Not sure if they still do.

 

If it's your personal trailer, install a permanent wireless camera on the back of your trailer. If you want to move it from trailer to trailer, use magnetic tape to hold it in place.Just make sure the surface is very clean before you attach it.Then see if it would be possible to have it broadcast on your i-phone. Stop at a radio shack or a spy shop like in Huntsville,Al. They might could give you some suggestions on cameras and what's available. Seems like it would be a good safety feature for you.

 

I thought about such and wondered if it commonly existed. But, unless it's your own trailer where it could be securely installed. I'd be afraid it would be too difficult to make it portable enough to be genuinely usefull and secure enough to not be stolen easily.
That was my thinking anyway.

 

It wouldn't (couldn't) work [well] in reality for a number of reasons. That's probably why no one has invested the money in such a system to date even with all the available technology. If anything might work, it would be a camera on a van/reefer trailer mounted on the top corner of the rear, right side, viewing straight down to view a sweeping view towards the front and along the rear of the trailer (about 270 degrees). It would somehow be need to be kept clean and undamaged from limbs. Wireless signal transmission at more than 40 feet may be a problem today.

 

I have invested the time and money on multiple methods. But each has logistic difficulties. If it is going to be moved from trailer to trailer easily then it still needs power. So it has to be either battery powered or you have to run a wire to it. Running a wire to it requires that you would have to secure that wire temporarily down the side, or under the trailer. Then you have to have a temporary mount for the camera. I have experimented with clamps and magnets.

There are wireless cameras, but they have a few problems too. One, they still need power. Two, they still need a temporary yet secure mount. And three they usually transmit the video using the 2.4 ghz transmission signal. That method of transmission is limited range and very susceptible from interference form lots of sources. So using a wireless that is 70 feet away from the base unit in the truck and does not fuzz out at the most inopportune moments has not happened. The only other form of wireless is to use a WiFi "IP" camera and they have a 1-2 second delay in transmission to the monitor. And they are much more expensive.

So far (for us anyway) the most practical method is to run a 65' wire down the length of the trailer and use zip ties to secure it. Then use a magnet mount at the back of the trailer for the camera. When you drop the trailer, you have to cut the ties and coil the cable and camera back up until you are ready to put it on another trailer. This type of system is relatively inexpensive because it is not a recording system and it uses readily available hardware.

So anyway, yes I have been playing with this question for about 2 years now. There is no great solution, but there are a few ways.

 

You can try something like this http://www.vuezone.com/shop/systems-and-cameras Instead of magnets or tape, how about little pieces of velcro? Velcro is cheap and no one will question why its there when you move from one trailer to another. Before you start to back up you just hop out and attach the camera.