I will take a case of bottled water and freeze half of them then pack them in with the food. They act the same as ice cubes but the best part is as the water melts you get a nice cold drink
well, I don't sleep in the FJ with the cooler with doors and windows closed - that would be 'helladumb', to reference Eric Cartman. The cooler is on the bumper rack or the roof or the ground when parked. No bears in and around Texas...ditto on the 'dry ice displaces oxygen' theory. I got some with a turkey once and fooled around in the apartment with it for a while. Carbon dioxide is heavier than air. That was the first time my ex pondered why she picked me out.
I freeze water in milk jugs. smaller water bottles would probably work better.