Did you figure this out? I have mine wired this way and it is extremely nice and useful. I also put a couple eagle eyes up at the tongue that light the ground as well. Really easy to make happen with a single 5 pin relay. I can draw it up quickly, but here is the wiring on the relay by pin.I am still figuring out the way I want to wire them but ideally I would like to have them just tied to the reverse lights on the truck with a secondary manual switch in the trailer so that they can be turned on at camp.
30 to the trailer lights +
87a to the truck reverse lights +
87 to trailer battery +
85 to trailer ground to switch
86 to trailer battery + to switch