In answer to your subject line question, I would propose cooler is anything that has a lower value than the current ambient temperature.
An unmutilated panel is by far the coolest thing. By unmutilated, I mean not having windows chopped into it.
Panel vans are cooler for sure, but considering we bought a converted panel I should say the other way round. Oh the joys of not really knowing what you are buying. If we had to do it over I'd get a proper one :
me too - a proper camper or a panel not a conversion. might have to go back to what it should be - a delivery van (but with a window in the sliding door)
as above really..if you want a camper get a camper etc..also I dont think there are enough panels around, so its a crime to cut windows in em....your money your choice at the end of the day