If you have any kind of a ranch operation, I'm sure you have an additional 12V battery around...and a battery charger. This will work for your current thawer that plugs into your cigarette lighter.
Get some clamps like those that you can hook a fuel tank pump or electric fence to a battery with and a female 12V power source end, and put those clamps on the power source, hook it up to the battery and vuwahllah, you will have a portable 12V power source for your thawer. I would doubt that you would have to recharge it very often, as my 12V will run some electric fence chargers for weeks on end.
If you view this link and look at the pc. on the left and the piece on the right, that is essentially what I used and built my stuff with. http://www.ppswest.com/cable.shtml