Wow, thanks for the details, landy! A bit advanced for me, i will have to reread carefully.
I use a usb fan in the window in daytime, in my current van, pointed in or out depending on sun angle, and that keeps things cool in about the bottom half of the van. Lower than the bed, but good for the daysleeping cat.
But without a roof vent I have routinely measured 110 degrees at bed height at 10:30am when it is about 60 outside and sunny.
New van already has a vent hole, which is unfortunately nowhere near the bed. I don't think I want to seal up that one and cut another...