How about this?

http://www.amazon.com/RoadPro-RPSL-350-Quart-Slow-Cooker/dp/B0013IR88A/ref=sr_1_1?ie=UTF8&qid=1422191657&sr=8-1&keywords=B0013IR88A

Already designed to run on 12v power. Truckers use all sorts of appliances in big rigs.