I installed a simple backup camera on my X17Z travel trailer last night that works through an app (GoVue) on my smartphone. It was $87 in parts plus generic wiring stuff I keep on hand. All I had to buy was the camera and a 2.4GHz transmitter with built in hotspot.
180*? CCD CAMERA with LIGHTS ~$40
Transmitter with hotspot ~$47
These same parts are sold with some other features for a lot more money elsewhere:
Same transmitter, lesser camera $120
Same camera, different transmitter $150
The camera and transmitter I bought both have compatible RCA video connectors and 2.5mm barrel power connectors. You can provide power to the transmitter through a red black power input wire and then plug the camera directly into the video and power out plugs of the transmitter. This works great for a backup camera on a car or truck where the transmitter is just inside the trunk or bumper near the camera.
I installed my camera 18 feet away from my transmitter on my travel trailer. I installed the transmitter in the battery box on the front where it's closer to the cab and my smartphone. The camera is of course on the rear bumper of the trailer about a 18" off the ground.
The camera included a 19 feet long male-male RCA cable, so I had to install a female-female coupler between the transmitter and the end of the cable. I did not use the power out of the transmitter to power the camera. Instead I tied both through a 3A fuse holder directly to the battery power. This way I can pull the fuse to kill power to the whole system. I plan to use the camera for backing but also for seeing behind the trailer while driving down the road. Many wire the transmitter and camera power to the reverse lights on vehicles, but that may require some boot and broadcast time before you can get the image up on your smartphone.
My phone connected to the wifi hotspot and the GoVue app worked right away for me, even in the presence of my internet connected home wifi. If you read reviews on these wifihotspot transmitters, most of the problems are folks who can't get their phones to stay connected to the WIFI_AVIN hotspot of the transmitter when an internet connected wifi spot is also available. I have had the "smart network switch" turned off on my phone for sometime, so maybe that's why mine works well so far.