The authoritative source that I use is Ugly's Electrical Reference by George Hart. There is an NFPA 70 series National Electrical Code published by the National Fire Protection Association. There's lots of factors including the type of cable, the outdoor temperature and what surrounds the cables (block them from dissipating heat).
2 Ga 75 degree wire will handle 100 amps of current in "free space".
1200 watts from a 12 volt battery is 100 amps - you'll drain that battery in about 40 minutes but the inverter will likely shut off way before then for not enough voltage. A typical starter batter is going to have 45-80 amp/hours worth of energy. I have some huge 105 pound batteries in my UPS system that have 120 amp/hours - they'd run your 1200 watt load for about an hour. 2400 watts from a 12 volt battery is 200 amps - you'll melt that battery in about 60 seconds as most starter batteries are not designed for the thermal load of a high discharge rate except for those few seconds to drive the starter motor. My UPS batteries are high discharge batteries designed for high current loads and will drive 475 watts for fifteen minutes - four times that out of a single battery is looking for boom-boom as the chemicals are going to generate huge amounts of heat and hydrogen (see Hindenburg).
A normal batteries life is greatly shorten if discharged below the 50% level. Deep discharge batteries are designed to survive the deep cycle. If you're going to be driving loads anywhere near that big I'd recommend running several deep cycle batteries. Of course with the truck running you can provide about 80 amps from the alternator meaning your battery is going to see a much slower drain.
To give you an idea of what 2400 watts is I have a watt meter on my home. When I'm not cooking with the electric microwave or running the AC my house is coasting along at 1200-1500 watts. That's a couple computers and monitors, external hard drives, a couple televisions, freezer, refrigerator, and a bunch of lights. In my truck I had a 300 watt inverter to provide power to my laptop GPS. Since then I've replace it with a 12 VDC to 18 VDC converter which drives the netbook directly with DC power just fine.
My UPS.
power inverter project questions
The motherload of free batteries.
power inverter project questions
