Now you are learning RV stuff. congrats.
If you really need to charge the camper battery from the truck, then you can use bigger wire, and not go through the thin wires around the fuse box. Make sure that stud #1, that some say is on all the time, is NOT on all the time or if you forget to unplug the trailer from the truck you could run down your truck starting battery. You need the trailer 12 volt power to be connected to the ignition somehow, and not straight to the on-all-the-time battery. Sounds like previous owner did some home-brew electrical kanosh work...
but you got it working, so good nuff for now...
Other option is a DC to DC charger.