When I bought my 2012 Ram 1500
(new) it was off a dealer lot and did not have the factory backup camera, so I ordered a PEAK unit from Amazon and installed it myself. It cost about $100 and mounts the camera on the rear license plate frame. I mounted the monitor onto the rear view mirror stalk, such that the monitor sits just above my rear view mirror. The camera is powered from the backup lights, the monitor from the truck's cigar lighter circuit (hard wired...I hate dangling cords.)
So whenever I enter the truck, start 'er up and put 'er in Reverse...the backup camera lights up and works perfectly. (It's a Bluetooth/wireless unit, but I extended the transmitter up into the cockpit area for good, strong signal.
I later bought the Airstream and really got tired of beginning a left turn to find some tailgating idiot passing me on the left.
So, I went again to Amazon, ordered another compete outfit, and installed the camera on the rear of the A/S, and having ordered an extension cable, ran the A/S camera cable to the front of the A/S and, using a 4-pole trailer connector, hooked it up to the rear of the truck, then carried the cable fwd to the cockpit just like the first unit.
I went to the autoparts store, bought a pair of common accessory relays, which removes the truck camera power from the backup lights, and puts the trailer camera onto a HOT power source, thereby turning the trailer camera ON.
Now, when I get in the truck and start 'er up... the truck backup camera lights up and helps be back up to the trailer to hook up. After connecting the trailer 7-way and 4-way camera cords, I go to the truck, push the switch (turning the truck camera OFF and the A/S camera ON) and drive off on a trip.
I can see whenever anyone is behind me. I can back the trailer into the campsite without assistance, and when the A/S is not hooked up to the truck, I have a backup camera in the truck.
The setup has "grid-lines" which clearly delineate the distance from objects and the width of the lane. I can tell exactly where I'm backing and how far someone is following. A little bit of labor... A lot of personal satisfaction...all for about $200.
Rear view mirror mount:
Rear view mirror extension:
Monitor Ram 1" ball mount (fitted this to back of monitor with small machine screw to clamp to mirror):
In this manner, I have only one monitor in the cockpit, and a spare monitor should I ever have a need for one, and very little money involved. The only drawback in my opinion is that there is not much distant-view discernable. A tailgater must be within 4 car lengths, or about 60' to really be noticeable.... but then, if they're farther back than that I see them in my outside mirrors, so there's no problem.
One more comment, If your vehicle has Bluetooth, you may want to turn it off while using the camera if they accidentally "pair" up with the camera. (Humorous comment: I've pulled into convenience stores and their own Bluetooth security cameras inside their beer-coolers have revealed stealth-cops hiding out behind the beer-racks apparently observing customers. HA!)
Hope this helps.