CCTV camera running 700 ft
Start at 24 volts (or more) and make a 12 volt regulator near the camera. To determine whether or not your primary supply voltage is high enough, take a meter to your regulator and check the input voltage while the camera is running.

And while this advice is simple and straightforward, I'm perfectly aware that actually doing it may be a challenge. While the regulator itself may be as simple as a single TO220 part (plus a couple of capacitors), you'll need to make a housing for it which is both weather-tight and insect-proof, which may well test your abilities.