Emergency hatches are much larger than 12x12.
Regardless, a thief is not going to draw attention to themselves by climbing up on top of the camper like a baboon to gain entry. They're going to pop the latch on the back door, clean and quiet, while looking like they belong there.
Unless, of course, you've set up your camper door like a Brinks armored truck. Then they KNOW something valuable is in there, and it will just make them more determined.
If you're that paranoid about "home invasion" perhaps you should consider getting a Brinks truck and outfitting it as a camper.