Let’s start by asking what type of van you have, a cargo van, or a window van with lots of opening windows in the back?
If you have a window van, you can get a 10" O2Cool 12v fan at Walmart for under $20, and crack your front windows, and open a back window and place the fan in it blowing outwards to draw air through the whole van. Much more effective than even 2 roof vents.
If you have a cargo van, basically you’re screwed. Even with two expensive roof vents, you’re only going to cool maybe the top foot of the van just a little bit. Cracking the front windows can help a little, but it will never compare to the ventilation available in a window van.
Your first choice should be to only park in the shade, never in the sun. In the shade, your van shouldn’t heat up to any more than the outside temperature. In the sun, it doesn’t take long for the van to heat up to 50°f+ hotter than the outside temperature.
Also available are cooling wraps & bandannas for dogs:
And cooling pads for dogs:
Similar things are also available for us people, plus there are also 12v heating & cooling seat pads and blankets available.
Finally, if you need to keep the whole van cool while camped out inside without the engine running, 12v swamp coolers work great and require very little power. In the desert, direct swamp coolers work best, in humid areas, indirect swamp coolers work the best. With my indirect swamp cooler, I can lower the inside temp to about 50°f below the outside temperature, and lower the humidity level by up to 70%. My home built swamp cooler draws under 2 amps @ 12v DC. It sits inside, in front of an open window, plugs into a cigarette/accessory plug, and has 3 speeds. On low, it draws under 1 amp, which is enough to maintain a comfortable temperature even when parked in direct sunlight. You do need your front window down about half way, so it could potentially be a security concern.
"The money to live comfortably is the best medicine in the world." ~ Road_Money