I have used a small 200 watt inverter plugged into the 12volt socket, then plugged my 200 watt slow cooker crockpot (which is also 200 watt) into the inverter and let it cook while I drove. My two crockpots both have regular 110 cords.
I used a regular 110 extenstion cord from the inverter to the crockpot, so I could set the crockpot in the sink on a piece of rubber shelf liner (so it wouldn't clang against the sink). I also stuffed a towel between the crockpot and the sink, so it was wedged in there. I used a piece of string to tie the lid down to the handles. My thinking was if I was forced to slam on brakes or make a weird maneuver to avoid a wreck, I wouldn't be hit with hot flying food. Also, should the crockpot slop over (some meals create a lot of liquid while they cook) then it would slosh down the drain.
If you have a 12volt crockpot, then it should work fine, just plug into the 12 volt socket. But for safety's sake, i would place the crockpot inside a tub or bucket or sink and have it secured somehow.
I only say this because when you least expect it, things can go wrong. I was driving about 35 mph down A1A in Florida. These two kids were on the sidewalk. Suddenly they just dashed out in front of me to cross the road! One kid ran across and the other hesitated before running. There was no crosswalk or intersection and I never expected them to do that. I had to stand up on the brakes so hard, I came up out of my seat. Luckily I didn't hit them, but I still remember the look of sheer terror on one kid's face in that split second when he realized he might be meeting his maker rather soon. It seemed he was only inches from my windshield, but I am sure it was much further.
I cursed up a whole new set of words I didn't know I knew, because their idiotic parents had not taught them how to cross a road safely. My hands and whole body shook from head to toe for about 15 minutes, I had to find a place to pull over, because I thought I was having a heart attack, but I guess it was just the adrenaline of such a close encounter.
Amazingly the crockpot remained in the sink, and the lid on the crockpot stayed on, but some liquid did manage to slop over the sides and spill down the side of the crockpot. It's the only time I've had it spill. Crockpot lids are not tight fitting on their own, I suppose to let heat or steam escape should it become too hot.