I checked out the specs on the smallest fridge, draws 4.5A max. That put max power @ 60W, but realistically, it will be closer to 15-20W factoring run time is not 24/7 and the power is probably 2/3rd the max.
That said 60W is just barely adequate if you are judicious with the temperature setting and you keep the fridge in the shade.
Check out
http://www.backup-power.ca/catalog/item/3393232/2970974.htm. They have some portable module models, not the best prices, but once you see some model#, you can do a web search for cheaper.
Also, a thought, Xantrex makes a XPOWER pack which is a battery, AC inverter and battery charger, all in 1 package. This may work for you because if there is some limited AC power available, you can utilize it, plus it's packaged for transport.
Note, this setup will not be cheap (lookin @ 1k), but you could probably get back most of the money invested when your done with it by selling it on ebay or craigslist.
Let me know how you make out.