I have a cheap wireless unit from walmart that has the small screen and license plate mounted camera. It works good for hooking up, but it is a small screen and has some interference in the signal/picture. I have been looking at cameras for the rear of the tt and have decided to go with a wifi camera. There are ones that can be viewed without an internet connection on any smart phone or tablet. Some can also be connected to a wifi network so you can log in from anywhere to view. You can even get ones that will pan and tilt controlled from your computer or smartphone, and have 2 way audio. I tried out the example that easyn has and you can mirror image both horizontally and vertically, pan, and tilt, scan, or view several preset positions that you can save. 4ucam has a basic license plate mounted wifi cam for around $100. View on your smartphone, tablet, or laptop. There is a short demo on youtube for 4ucam.
I would like to get the ones with pan/tilt. Our tt is parked in the grass next to the driveway, and I keep the shore power plugged in all the time, so if I used a camera with pan/tilt I could view our driveway, garage, and house on my phone or laptop from anywhere. Most of the cameras that I have seen have a motion sensor that can send you an email with pictures if any motion is detected.
To use as a backup camera, you will need to be sure that the camera you get can connect directly to your device without an internet connection, as most cannot.
Another option I have considered is a wired camera with the monitor that mounts onto the rear view mirror. Some of those have a fairly large screen. It would cover the compass/temperature display on my mirror though.
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