A regular portable cooler really helps when you go food shopping and it's hot out and you are a long way from the cg or you want to stop on the way back to the cg or want to shop some more. Also good if you have a separate tow vehicle and want to go for a day trip or picnic somewhere.
We have a 12 volt cooler that is very handy in the heat of the summer for keeping bottled water, pop and bulky fesh stuff like lettuce cool. Got our Coleman one at Walmart. Only thing about these is that they cool your items to about 40 degrees F colder than the ambient outside temp. and can't be adjusted. This means that when it's really hot out, your drinks may not be cool enough. And when it gets cooler out, stuff inside can freeze. When it's hot out, you can keep the cooler in the shade which helps. We're going to add a 12 volt outlet in the bed of the truck so it will keep stuff cool while travelling. Many brands of these out there. I thought there are none out there that are temp. controlled, but it looks like Koolatron has one.
Here's lots of good info. on Koolatron:
Koolatron cooler info.