Every system I have seen is wireless. You have a separate monitor that you put on your dash. It isn't powered by reverse but rather is on all the time.
When they say pre-wired it means there is a power cable prewired to power the wireless camera.
There isn't a way to send video through the trailer's 7 pin connector to the head unit in your truck which probably has a backup camera. When you put it in reverse that will still work as it does now and you will get a nice image of your propane tanks.
Edit: As far as other options. I know that Garmin makes a camera (wireless again) but the difference is rather than have a dedicated monitor for the rear vision like the Voyager, the monitor is the GPS Screen. I don't have it but I know it is another option. I don't know if the camera is powered by battery or off the prewire that Jayco has in the camper. Here is a link.
https://www.amazon.com/Garmin-760LMT.../dp/B00IUO7PHE