Thermoelectric coolers are not very power efficient and actually produce a lot of heat, which would tend to heat the space in the back of a vehicle and would raise the temperature in the vehicle boot even higher. Conventional compressor technology refrigeration would give the highest efficiency and keep any stored food at the highest temperature differential to the ambient temperature. Again a solar panel (i.e a 40W semi flexible marine PV panel), Solar Charge regulator, light activated relay and SLA Battery (i.e. Xantrex 300, which would a useful vehicle battery backup anyway) could be setup to power the a refrigerator such as the Waeco CDF-18 mini freezer/refrigerator. Food and water could be stored in Vacuum Flasks within the refrigerator.

http://www.minicoolers.co.uk/products/waeco/cf18.htm

As you can see it is not a cheap solution to a difficult problem.