I just bought one of these for the tow vehicle.
Wireless Car Rear View Kit 7" LCD Mirror Monitor + Reverse Back up Camera USA F | eBay
It works pretty good and there are 2 inputs. I did use both inputs on the other tow vehicle I had. There is a button you press on the monitor to switch between the two cameras. I absolutey recommend throwing the wireless portion in the garbage. I just ran an rca cable from the back of the trailer to the front and same in the tow vehicle (2 for you) and hook the wire up when hooking up the trailer.
The wife absolutely loves the fast that when looking in the rear view mirror when towing it looks like you are looking out the back of the trailer. Absolutely no guessing if you have cleared a vehicle you are passing and you always know when you are being tailgated.
When used as just a mirror it is not great but does the job. I usually just remove it and put it in the trailer when not towing but to each their own.
Also, I would hook the power for this unit to a switch. Mine just turns on with the parking lights
as in I would never tow without being as visible as possible. I leave it on the whole time I'm towing down the road. Towed from home to the farthest point east in Canada and would never do it again without this set up. For you, just use a switch or whatever as you may not want to leave it on when not towing and just using it as a backup camera on the tow vehicle. I did run the rca cables as you suggest.