My tiny bus has 2" of foam insulation top, bottom and sides and double glazed windows except for the windshield. Based on simple calculations, a 6K ac unit should have been enough to cool the interior from 105 degrees F to at least 85 degrees. The test of this came on an asphalt parking lot at a Camping World in Bakersfield, Ca. in July. The asphalt was 117 degrees and the air temp was 107 and the sun had begun to set. We smugly closed up the bus, pulled the reflective window shades, flipped on the ac and waited for the bus to cool down--it didn't, rather it turned into a sauna. Dripping with sweat I told my wife that I thought I must have miscalculated the ac requirements when it struck me that if I opened the two small zippered windows in the pop top, it might help. It did, and within 15 minutes the interior had dropped to 85 degrees and finally to 80 degrees after a bit more time.
I guess the point here is that you need as much ac as you can support and a mechanical way to rid your bus of the hot air that gathers near the roof.