The trailer plug wiring provides 12 volt charge, but really just a light trickle that keeps it topped off, or eventually can recharge it back on long drives.
I posted a thread on this topic somewhere on here how I ran a separate charging circuit to the camper batteries.
In short, I pulled positive 12 volt power off of the truck's starter motor. It is directly connected to the truck battery and always live. The starter motor is only used for starting, so that heavy gauge wire is unused 99.999% of the time so can be used to charge the RV battery. Doing this saves from having to run yet another wire to the battery and pass it through a crowded engine bay.
From the starter motor I ran the wire into a solenoid, which I controlled via switch in the truck (but it can be turned on with the ignition if you prefer that. Because of solar I rarely needed to charge the RV from the truck, so the switch was off most of the time).
Then the wire went up through the truck bed and made as direct a route to the camper battery as possible. I made a new connection point on the camper wall for the charge wires, so this was another connection to make when loading the TC.
I used a big Anderson plug to connect the camper to the truck, but whatever works for you as long as it is safe and can carry the amps is fine.
I also did the same thing for trailer charging while I was down under the truck getting dirty. Instead of the Anderson plug I used a 2-pole trailer plug n socket (similar to the 7-pin) to connect the trailer to the heavy gauge charge wire, so the charging wires off the 7-pin were put on yet another switch so that could be turned off when the big plug was being used, or maybe they were on the same switch, I cant rember now.