On most of the wired versions, the wiring instructions tell you to wire it into the back up lighting circuit. They don't tell you, wiring into another circuit will give you a choice as to when the system is "on".
I've installed them on 3 - 4 of my vehicles. Depending on ease of getting at the wires, I have used auxiliary power outlet circuit, cigarette lighter circuit, running light circuit, and made a plug that will plug into the power outlets.
Currently I have camera systems installed in two trucks. One is setup with camera being on, any time the ignition switch is on. The other has a plug that I can plug into the lighter ("on" when truck is "on") or auxiliary power outlet, which is always on. I have forgotten to unplug it from the "always on" power outlet, left the truck sit for three days, and had no problem with dead battery.
It's pretty much, run the wires to the fuse box and select any circuit that suits your intended use, or wire it to a plug in harness (home made) and plug it into an outlet that suits your needs.
I wired trucks and camper's (TC and TT) individually. There are all kinds and sizes of CCTV cables and plugs on eBay and Amazon. I have cameras mounted on rear of truck camper, on rear of travel trailer, and one loose, that I use for hitching up trailers or loading the TC. My monitor is one that clips onto the rear view mirror. I bought the cameras, monitor, and cabling, individually. Couldn't find a kit that fit all my different uses.