Not having run careful tests, here's my hypothesis:
1. A van gets hot inside, white not as much as black.
2. Heat couples up from the van's interior, through the highly conductive metal roof onto the rear of the panel.
Theory would say that a black van would be worse for a panel's efficiency.
The reason that you found it hot working next to a white van supports this hypothesis. The white van is reflecting heat away from the van to where you are standing.
1. The concrete is tied to the large ground heatsink, which is able to suck heat from the panel.
2. If you shade a piece of ground, it will be cooler than the surrounding ground. That's how @orton
's floor vent scheme works.
This is why it's more dangerous to leave a dog in a van than lying on the ground outside.