I agree with super insulation. A little bit of insulation won't help much. I have about 3 inches of rtech foam insulation on the roof. It helps in both the winter and summer. Once I super insulated my van I no longer needed a heater in winter (lowest outside temp I seen is about 32f, inside of van is over 50f with no heater). In the summer my small swampcooler keeps the back of the van cool with less than 2 amps of power. Even without the swampcooler the insulated section rarely climbs over 100f, the front uninsulated section of van reaches 150f.
Without insulation if its 40 degrees outside, its 40 degrees inside your van, it was brutal living in those conditions, I needed 2 sleeping bags and some blankets on top of them. Temperatures above 50f, I didnt even need a sleeping bag.
Cargo van with bare metal walls would be the best for insulation, a hotglue gun would make quick work insulating it.