Steve,
You can add 12 VDC to a pin for charging your trailer battery during the tow. NOTE #### the plug used for this on trailers is about 3" in diameter and not a 1 to 1 1/2 small plug used for standard towing applications. The smaller plug is not intended to carry the amperage and will not be wired on truck side for this application.
The better way is to get a RV or Trailer plug if you do not have one rated for amperage. See
http://www.rverscorner.com/wiring/7pole.html .
Use proper wire gauge for power and ground directly to a battery source not fuse panel. Also install a battery ISOLATOR
http://www.pplmotorhomes.com/parts/r...y-isolator.htm that will keep trailer from draining truck battery when parked.
HAVE FUN !!!!!