Based on my street bike experience with heated gear, I cannot imagine a better way to keep warm at 30F on an ATV. There are two main power sources for heated gear: the vehicle's 12V electrical circuit and portable rechargeable batteries (usually 7.4V Lithium Polymer). I am using both.
If the alternator of your ATV can pump out enough amps for the heated gear, in addition to the current consumption of the vehicle, like most big touring bikes and modern sportier bikes do; 12V is usually the preferred way. Rechargeable battery types do not tax the vehicle's electrical circuit and they work off the bike (ATV) too, until their batteries deplete. You can swap batteries, though.
There are very good jacket and vest liners on the motorcycle accessories market, as well as gloves, socks or insoles and pants. I suggest you determine your vehicle's own current consumption first, then your alternator's power and see how much current you can spare for electric clothing, taking into account some losses. No electric circuit is 100% efficient.
Then, decide on what to wear and how to control the heat setting of each peace. Usually, dual controllers adjust the heat setting of a jacket liner or vest liner plus one additional item (gloves or socks or insoles or pants). The third item needs to be controlled in conjunction with the second one and this is not a particularly comforting setup most of the time. The controllers can be wired or connected via Bluetooth for ease of use.
There is a lot to be said on this subject but I hope the above will give you some food for thought.