The charge line will keep power to the trailer while the tow vehicle is running. Not only charging the battery, but power to the refrigerator. Not sure what else would be drawing power while towing. I find it useful, especially if we stop on a rest stop and want to run the furnace at night. I know that when I shut the truck down, the trailer battery is charged and will run the furnace for the night.
As far as the reverse line, unless you have reverse lights, or tow a trailer that needs the reverse signal to release the brakes when backing, I wouldn't worry about that. My buddies large boat trailer needs the reverse circuit. None of my RVs have used it.
You are correct, for the long run of the charge/hot wire from under the hood, you want a heavy gauge wire to carry as much power without losses. I used a heavy duty self resetting breaker, not a relay when I wired my Explorer from 4 pin to a full setup. Fortunately, my F250 can factory wired for towing, including a connector for the brake controller harness.
2013 V 3105
2016 F350 King Ranch CC 6.7 diesel dually