If you don't have reverse lights on your trailer (very few do, it seems) you could tap into that wire (pin 7) to use as your camera circuit, perhaps. You would need to modify the wiring on the truck to do this, and it wouldn't be something you could move from vehicle to vehicle, plus the trailer wiring would be "special". You would need to cut the wire from the reverse lights and put a new wire connecting that pin to your dash switch.
Alternately you could perhaps just switch the pin 7 wire so it still acted as a reverse light unless you flipped the switch. Then your camera would be on either when your reverse lights were on
or when you flipped the switch. This would be best done with the switch driving a DP relay to disconnect the reverse lights when you switch to activate the camera so nothing weird happens. Reverse wire to NC terminals and your camera power to the NO terminals and the switch drives the relay. At rest you get reverse lights and when in reverse your camera on the trailer would be on. When you flip the switch, the camera would be on, independent of the reverse lights. This is assuming you do not have reverse lights on the trailer (or don't care about them). If you do have them, then this solution would turn on the reverse lights on the trailer when you flipped that switch, but the truck's reverse lights would not be on (unless you were in reverse...)
Depends on how complicated you want to make it. I like that second solution, but then I like things complicated
If you switch #4 as you suggested, you will not be charging the trailer battery when it is off, which sounds like it would be most of the time.
