OK, I found a great wireless solution for rear cameras on the back of a travel trailer, with a digital encoded signal that rarely if ever drops out. Here it is:
Boyo VTX300D Vehicle Camera https://www.amazon.com/dp/B00HJOSV3I?ref=yo_pop_ma_swf
Basically NOT a camera, and the lone picture is terrible, but the transmitter-receiver pair this kit is made up of work great for getting a clear composite video signal from the back of the trailer to the cab of the truck. No camera is included, but any composite video (yellow RCA plug) camera will work with the transmitter, and the receiver in the cab has the same composite video output to your radio or monitor.
A couple of things I should mention...
1. If you already have a backup camera on your truck, the best way to connect the Boyo receiver is to use a two to one composite video switch, so that you can switch between the rear bumper camera and the trailer rear camera, sharing the same monitor (rear view mirror or dashboard or radio). Once you have the trailer hooked up to the truck, your bumper camera is useless (except to see that the trailer is still attached, blocking the entire view), so you switch to the trailer camera.
2. You need to mount the receiver up above the pickup bed and all that metal. Mine is up on the left rear cab pillar, by the rear window. Great signal from the transmitter on the back of the trailer right through the trailer to the back of the cab.
The transmitter connects to a 12V or 24V power source (can be the reverse light on a vehicle but not to leave it powered up on the back of your trailer), and has two plugs out to the camera, one video and one providing power to the camera via some standard power plug. The camera I bought had the right power connector so it hooked right up, easy peasy.
The Boyo description on the Amazon website says it is waterproof... Well, kind of. The TRANSMITTER is in a waterproof silicone sleeve like a little scuba suit, but that will dry out and crack in the sun and heat, and once there are cracks it will leak. Only an idiot would leave it dangling in the sun, so plan on either cutting enough of a hole in the wall when locating a power wire, or mount it in some kind of plastic (not metal!) protection box. Look in the outlet and yard power area of your hardware store - there are a bunch of potential protective boxes and outlet covers you can use to protect the transmitter from the sun. I also suggest sealing any holes, seams or screws with clear silicone sealer (paint aisle).
The Boyo transmitter-receiver wireless solution above is independent of the camera you use, so long as you use a camera which has a composite video connection (yellow RCA plug). For my camera, I chose the Snanshi unit below:
SNANSHI Truck Lorry Pickup Bus Vehicle Caravans Backup Reversing Rear View Waterproof Camera with IR Cut LED Night Vision DC 9V - 35V Wide Voltage https://www.amazon.com/dp/B00YS80JNS?ref=yo_pop_ma_swf
It's an experiment, so far I'm liking it. Two minor issues.
First, the angle is adjustable, but even at its max upward position it is angled down at the bumper so that I can't see the road behind the trailer. This is fine for making sure you don't run over anything, but I needed to see any tree branches that might impact the trailer. I ended up attaching the camera mount to a cheap piece of galvanized metal plate from a hardware store framing aisle, which is then screwed to the trailer rear wall up high. I can easily bend the metal to achieve my desired angle.
T second issue is that the low light function, while automatic, requires the camera to switch to a black and white mode, which drops the video for 1-2 seconds. No big deal, except that around twilight it changes back and forth as the trailer moves or the light level changes, and it's off more than it's on. If you are trying to back up your trailer at the same time, it would be very inconvenient.
On the plus side, it has IR LEDs and is very bright at night, once it settles into low light mode. Also, the built-in sun shade is good for keeping the direct sunlight off the lens except for sunset and sunrise. And at under $30 it's a great deal.
So pick whatever camera you want (with the yellow RCA composite video plug), and you can use the Boyo transmitter-receiver pair to get a clear video stream from the back of your trailer to the cab of your truck. I definitely recommend this Boyo wireless solution for long rigs with 5th wheel or travel trailer.