If you plan on staying in your van during the day, you need massive amounts of insulation. Several inches thick all around the back of your van, cover the windows, separate the driver compartment from the back of the van.
Then get a swamp cooler, swamp cooler doesnt work great in humid conditions but its better then just a fan. Hot outside passing through an evaporator pad will cool down. On hot humid days when I don't think I'm getting any cooling if I turn off the water pump, air going through a dry evaporator pad will feel like a hot furnace. I can't spend even 10 minutes in the back of my van without the swampcooler running, I always have spare fans/water pumps to make quick repairs if the cooler stops working. Swampcooler even in florida will take the edge off the heat, and the foam insulation will do the rest.
You can build your own swampcooler, a 120mm case fan, a water pump, plastic container, pvc pipe and evaporator pad. Use the celdek evaporator pad, I found that works the best in all conditions. You can find most parts at home depot. Depending how many fans you use it can use less the 2 amps of power to keep you cool.
I park in the sun every day, no matter the weather, and stay in my van all day with just the swampcooler running.
rtech foam insulation, straight ahead is the driver area behind the foam insulation. Every inch of the back of the van has several layers of rtech foam insulation, the roof and windows have up to 3 inches of foam.
temp difference between front uninsulated section of van and back of insulated van. 94 is high but livable, its cooler right next to swamp cooler. I made improvements since this picture, I been able to get overall temp down to the 90's. Without the swampcooler the back temperature can get as high as 110 degrees. with no insulation, it would be 148 degrees in both sides.