Check out page 38 of the PDF...
It calls for a 10 Ga wire for both the charge line and the ground. Make sure you have this on the truck side, too. Chances are you won't. Also check the circuit breaker size. This is not the best way to hook up a second battery, but it should work, as this is how the trailer plug is supposed to work to begin with. I have had fuses and/or circuit breakers trip when trying to charge a too-dead trailer battery. In other words, have a way to check the charge of the aux batttery as you use it.
As far as a harness is concerned, two wires, a plug, a fuse or circuit breaker, and two eyes on the end is pretty easy. Add a couple of fused recepticles, and you are in business.
Oh, be sure that battery is secure!
Good luck, and have fun camping this weekend!
03 Taco (Gone, and missed...)
88 FJ62 3FE, auto, etc. Sold
07 DL650, lightly farkled