Adding batteries to a cargo trailer?

I have had them in my last couple of trailers. Not much too it. Find safe place to mount them and wire in. Most newer rigs have a power wire to the 7 pin trailer plug. On the trailer I have now there is a solar panel on top to keep them topped up,plus an converter/charger. The PO installed an RV heater and we have ran it all weekend with batteries never dropping below 12.6 or so.
For what you want it's basically mount boxes & wire in to existing power lines..........

I actually put LEDs inside my trailer and wired the emergency brake battery into the interior lighting circuit. The inside lights don't use much of the battery and the truck charges the battery back up when on the road. I know that probably doesn't sound like the safest thing to do because you're using your emergency brake battery power but it has to be better than driving around with a bad/dead emergency battery like I was before unknowingly. Running your lights off of it is a good check to know that the battery is still good. The normal little battery probably wouldn't be enough to run all your exterior lights for very long though but you could replace that emergency battery with a larger battery capable of running everything.