12 volt appliances suck, basically. They're designed to draw their power from a cigarette lighter plug, so they don't use enough power to actually heat anything quickly.
I had a 12 volt coffee pot once, and after discovering that it takes the damn thing 40 minutes to brew a pot of coffee, I couldn't even give it away. I ended up washing it and putting it back in the box and I left it on the table in my company's drivers lounge with a note on it that said "FREE - nothing wrong with it, .. it's just too slow - 40 minutes to brew a pot of coffee". The boss said it was there for 4 days before someone took it.
A friend of mine has one of those lunch box heater things. I really think those things are designed not to heat food, but to simply keep it warm for day-cab drivers who cook their lunch at home in the morning. But my friend uses his every day for cooking, but it takes it 4 hours.
A 12 volt crock-pot however, seems to be a good idea because crock pots are SUPPOSED to cook things slowly, so it doesn't make a difference that it is designed to operate off of a cigarette lighter.
I don't think I've ever seen a 12 volt crock-pot so I'm wondering if they're made of glass or if they're plastic with stainless steel inside or something. I don't like having glass items in the cab of the truck.