So, I checked the (Furrion) system the trailer is prewired for. Very expansive (over $400!!!).
Since I also wanted a hitch camera, I looked elsewhere and bought a cheap Yakry Y23 HD 1080P ($180) wireless system with 2 cameras.
The dash unit needs power (currently just cigarette lighter plug/switch that comes with the system, and one of the 2 included dash mounts).
Hitch camera mounted to licence plate screws, wired into the hatch and spliced into the back up light (my Touareg has backup light in the hatch). It camera and monitor turn on when shifting into reverse (1-2 sec delay).
I mounted the trailer camera bracket to the existing cover (2 screw holes, one hole for cable). Cut off the Furrion round connector. I had previously asked if someone here knew what type connector it is (to get a matching one) and how it is wired, but did not get an answer.
Connected Red and Black wire. Not working. Turns out Furrion/Forest River don't use the common sense rule (red is positive, black is negative). Fixed the wiring accordingly. Used the connectors from the kit and assembled. The Furrion is simply spliced into the trailer lights (so power only when driving lights are turned on).
Working fine now.
Only issue: need to manually (push button on back of dash unit) to switch between hitch camera and trailer camera.
To turn the trailer camera on, the headlights have to be on (so the trailer lights are on).