Scout, if you're in the army, you need to go the motor pool and get them to make you an adaptor box/cable for your 12 pin cable to a civilian 7 pin cable. I pull a civilian trailer that I changed the bulbs to 24 volts on and added a second power lead from the truck to supply 12 volts to the battery charger for the breakaway system (probably the battery you're speaking of right?).
I have a box I mounted on the trailer, depending on the setup, you might want a box you put in the bed of the HMMWV, you run a 12 pin cable to the trailer connector on the back, and plug the 7 pin plug into the box.
To get 12 volts out of 24, you'll need a 24-12 volt converter. I'm not aware of a particular NSN for such an animal, but I do know I have one from a British mfgr. There very well could be such a thing for US MVs. That would go into the box and drop a 24 volt (split if off of the tail light feed)
feed to 12 volts which you'd send to the trailer. You would likely have to run with the lights on always to power the battery charger but it would work.
You could get a 7 pin receptical added to the back of your HMMWV instead of a box. But the box would be portable and it could be moved from HMMWV to HMMWV (or even behind a FMTV or Deuce). Put some magnets on it for "anchoring".
For the lighting, either swap 12 volt to 24 volt bulbs OR get some dual voltage LEDs.
Hmm, I just thought of something, does your rock wall trailer have electric brakes? If so that presents an additioanl issue. You need to provide 12 volts on a resisted (basedon braking force) design for the brakes. That needs an elecronic brake controller.
