one thing you can do is put a vent/fan on roof and remove the cover, next if you got a roof rack attach a solar panel over the vent. This well keep rain from getting in you van plus still offer ventilation. Anyone passing by wouldnt see the vent.
On my van thats what I had to do to get the solar panel to fit, but i didnt have a roof rack I just use 4 long L-brackets to install the panel, in the 4 years I had the roof vent uncovered, I never got water in my van when it rains. On cold days I just cover the vent from inside to keep the cold air out.
This will also solve your problem of power for your fan, a 60 watt panel gives you 3 amps, 80 watts 4 amps ( which is what a fantastic fan uses). On cloudy days it won't be hot so you wont need to run your fan at high speed. You do need an aux battery to run something as large as a fantastic fan.
But the problem I see with the vans you mention are all the side windows. Those vans are all windows and thats where all the heat comes from. My advice is get some 1 inch rtech foam insulation (from home depot) and put them on the windows. In california where the weather is mild, it gets hot real quick (hot like unlivable) as soon as the sun comes up. The roof vents did nothing for me. Foam insulation and swamp cooler was the only thing that actually work.
This is a picture of the thermometer on my astrovan. 141 degrees is in front of van, 98 degrees is in back of the van. Without the foam insulation/swampcooler both temps would be the same 141 degrees.