We have had a 40-quart Coleman thermoelectric cooler for as long as we've had a TM. See it (but don't buy it) at
http://www.coleman.com/coleman/colem...ategoryid=8570
We got it from WalMart for a much lower price than on the Coleman web site - I think about $50 including an AC adapter. For us it has worked very well. We carry it in the tow vehicle as we travel, plugging it into the cigarette lighter, and then bring it into the TM when we reach the campground. It is a nice cooler just by itself - you could chill it with ice if you wanted to, but the plug-in feature is nice.
Advantages: Inexpensive. No need for ice. No water everywhere. Nice cooler all by itself, even if you don't have power. No moving parts except a small fan. No need for ice. No need for blue ice. No ice, no water, nowhere.
Disadvantages: It takes about 5 amps at 12 volts to run it, so it is not workable if you are dry camping. And, it cools the inside of the box to about 40 degrees below the temperature outside the box. If it is 100 degrees outside the box, it will be cool but not cold inside. If it is 50 degrees outside the box, things will (eventually) freeze inside it. We haven't found this to be a big problem - we store our "fussy" things in the TM refrig, which is temp-controlled, and use the cooler to augment it, choosing things that won't be hurt by the expected temperatures on that particular day.
Not everyone has had good luck with it. But I think the biggest problem has not been with the cooler itself, but with the kinda lightweight cigarette lighter outlet in the TM. If you search this board on the term "thermoelectric " you will learn more.
Aside from thermoelectrics, there are some small coolers that have an actual refrigerator/compressor mechanism just like your home refrig. I have had NO experience with these, but would like to hear from anyone who has. Kinda spendy, I think.
BTW, if you buy a thermoelectric cooler, BE SURE that it includes the AC adapter. You definitely will want one, and if sold separately, they are VERY expensive. On the Coleman website I linked above, the cooler itself is $78, and the AC adapter is an additional $32. Yikes! Go to WalMart!
Finally, some folks have had good luck with a small "dormitory refrigerator", about the same size as the main refrig in the TM.
Disadvantages: requires AC power to run it - no DC power option. Not really portable.
Bill