Yes, I've done this with a 10amp+ charger if I'm running the refer over the night before we leave and must keep the unit close.

Simply push a battery charger through the two proper terminal on your trailers 7 way Pollock connector.  Specifically, Pin 1 [white wire] is common ground.  Pin 4 [black] is your battery charge.

If you jury rig the setup just be careful you don't accidentally short the leads -- the best scenario is you'll blow a fuse in-line with the battery; OR if you don't have a fuse, you'll have some fireworks.

As far as keeping the battery topped off (we store the TM in our garage) I use a small 12v DC wall transformer which plugs into a special jack in the side of the battery box.  The wall transformer delivers approximately 200mA to trickle charge the battery.