Living in a small town in Wyoming, I often face limited choices at the supermarket. We have two, but they're both small and they both tend toward the lowest common denominator. So often I can't get things I want here, or I can but it just isn't fresh.
The solution to that problem is, of course, to shop in the Big City. The best time to shop at a Walmart Super Center is at 05:00 when it isn't crowded. But how do you get refrigerated or frozen stuff home?
The solution to that problem is a refrigerator. I'm not talking about a cooler filled with ice. Ice melts, coolers leak, etc. A cooler cannot guarantee a given temperature. Nor do I mean a cooler fitted with a thermoelectric cooler. They get at best a 40° Fahrenheit difference in temperature (often less), which makes them useless if the temperature in the car is above 90°. I'm talking about a real refrigerator with a compressor, temperature controls, and lots of insulation.
I'm talking about the Model FP430 EdgeStar Portable Fridge/Freezer. I've had one for a while, and it works quite well. I buy a half of a buffalo from time to time, and once in a while I need an overflow freezer. My EdgeStar has done that duty very well. When it's not doing that, and temperatures are warm, it sits in the back of the car.
I've had one problem with it. Hidden in the 12 volt plug is an odd size fuse. I managed to blow that fuse, and could not find a replacement. The workaround is to splice a blade fuse holder into the line, and replace the fuse in the plug with an aluminum dummy.
I have a 2009 Subaru Forester, and I'm very pleased with it. It has three power point (cigarette lighter to us dinosaurs) sockets, one of which is in the cargo bay. That one provides up to 20 amps, which is what the EdgeStar's fuse is rated for. The power points are controlled by the ignition switch, which is a good thing: you can't run the battery down by leaving the fridge plugged in. The bad news is that you really can only run the fridge from the car when the engine is running, i.e. when you're driving.
There are two ways to work around the latter. The simplest is to plug the refrigerator into the mains when you aren't driving it. That helps conserve the car's battery. That's great for pre-cooling before you load up and depart. It does not work very well in a Walmart parking lot. Nor does it work when you go camping. At least, not where I go camping.
By the way, if I do plug the car into the mains for any reason, I loop part of the extension cord through the driver door handle or over the driver's windshield wiper so I don't forget it's there three days later….
The solution to that is a deep cycle battery such as those sold for marine and RV auxiliary power, or for solar power installations. You also need an alligator clip to power point cable, which most automotive stores carry. Add a fuse to that cable. You should put the battery inside a battery container, partly to contain leaks, but more to have a cover on the battery's terminals.
For charging the battery from the mains, a 1.5 amp trickle charger does just fine. For faster charging, get a higher capacity charger. However, the faster the charger the more likely it is to damage the battery by charging too fast. I find a 1.5 amp charger will recharge my battery in 24 to 36 hours.
Now, should you live in an area prone to power outages, you have a battery backed up refrigerator or freezer. The fridge will automatically fail over to the battery should it lose power from the mains.
You can get solar panels. A 5 watt panel does not require a charge controller, but it simply will not keep up with the fridge. 60 watt panels are large enough to be awkward in a car, and require a charge controller. But one such should extend the battery charge for quite a while. You want a heavy duty extension cord long enough so you can park in the shade but still put the panel out in the sun, but not so long that the voltage drop kills you.
The next trick is charging the battery while the car is moving. Get a vehicle to vehicle battery booster cable. These go from power point to power point. I use a Black&Decker Simple Start, catalog number BBC2CB. It has smarts on it, and delivers about 8 amps. Buy a twofer, and then you can leave the whole setup plugged in, and you don't have to muck with it while you are traveling or camping. They also double as part of one's emergency kit.
I took this setup with me last October to visit a friend who lives off-grid in Arizona. I was there for three days, and didn't run my car engine. Temperatures in the car got to over 95° daily. The fridge was still running from the deep cycle battery after those three days.