I think the scoops would be fine if there was something that was drawning the air in either by a fan/blower or a natural flow. By having an opening of the air to go out the back of the van. have you tried opening the rear hatch a 1/2" or so to see if the air woud flow thru the bus any better. I would say a pop out window on the rear hatch would get the air flowing pretty good thru the back.
Just my opinion mind you.... it just seems the air either needs assistance getting in, or a natural source to get out. Kind of like the wind blowing into your bug thru the vent windows in the summer. then you pop open the rear pop outs and give that air someplace to go and whoosh its much cooler.
I'm no scientist, but I figured it is worth a try
