To address your question specifically, a 24-12V converter would be used to put 12V to the back of the truck such that it would draw from both batts rather than just one. That would then actuate to the civi trailer connector via three 24V relays for Left stop/turn, Right stop/turn, and tail lights. If you're running trailer brakes, a 12V feed would also go to the trailer brake controller, with another 24V relay to trigger the 12V brake switch signal to the brake controller.
Not that it's the right way to do it, and given long term use, it's not ideal, but I ran a 12V wire from the low battery to the back of the truck, then actuated the 12V to trailer turn signals/brakes via 12V relays that I had laying around (driven by 24V of course...) It will draw down the one battery if used A LOT, but for my daylight use (no tail light feed) it gets me by with brake lights and turn signals. Incidentally, the 12v relays draw a bit more power running on 24V, but it's still small. Seems OK for intermittent use so far, but they click LOUD!!
They do get pretty warm if you leave them on continuous, which is why I didn't run a tail light relay. Couldn't find 24V relays, too cheap to buy a 24-12V converter.
YMMV.
