MIke,
You have the problem 90% solved already by the description in your post. Your refrigerator is fed by a 12 VDC power connection. You can tap into those 2 wires to get the power for your 12VDC outlet.
The benefit is that you don't have to cut the skin of the A/S....just mount it INSIDE the refer door! It will also be out of the way. Be sure to get a weatherproof outlet.
www.powerlet.com has lots of those mainly used for motorcycles but will work perfectly for your application. Your local auto parts store might have one also.
At the bottom of the fridge, you will find a white plastic terminal block with 2 wires coming in and 2 going into the refer control board. The 2 input wires should be red and white, with red + and white -. Just loosen the retaining screws and add your new wires for your new outlet and re-tighten.
I would suggest that you pull the fuse for the refer BEFORE doing this to avoid any shorting of the + lead, which would blow the fuse anyway.
Good luck!