The problem withthat Avanti unit is that it is a thermoelectric model. This may be okay to chill some sodas, but it won't make anything "cold" in 90 degree weather by any means. Only a fridge with a danfoss compressor will give you real ice-cold in any temperature while being able to run on 12 volt. This has been bugging me for a couple of months now, and then best solution that I was able to come up with in regards to performance vs. bottom line dollar is to go with a cooler-type fridge from Waeco/Dometic. I am not sure how they are connected company-wise, but their offering appears the same.
They go from small to large.

They can't be installed, but can at least you can strap them down, leave them plugged into 12v for an awfully long time without worrying about amperage draw, and I find the top loading a benefit. They do have one that is half freezer and half cooler. Good for sodas and ice cream in the same cooler, but I think it is expensive. The smaller one, the cf-18, is probably the one I am going to go with. Has a good capacity, and is right around the $300 mark. Good luck.