I've looked too. I would love to see a step-by-step to help fortify my confidence. Even a few pictures of a finished job would be helpful.
Why are you interested in adding a second battery? What do you want it to do for you?
Before you dive-in you might want to consider replacing your stock converter so that you won't be as likely to overcharge and ruin whatever battery system you decide to install. This is also a great time to plan to install any battery monitoring equipment that you might want.
Depending upon the amount of room that you have available you might consider purchasing one large battery instead of two smaller batteries. Perhaps one group 31 (or larger) instead of two group 24s? Here is a chart that I find useful when I'm comparing different combinations of batteries:
Lifeline Batteries - Marine & RV Deep Cycle Batteries
Also note that AS requires that the tongue weight never exceed 15% of your GVWR. +200 lbs. of batteries installed on the tongue is probably not reasonable on our little trailers.
I hope you'll share any good information that you find while you look into this.