Foam insulation. Lots of foam insulation. That and a small swamp cooler is how I stay cool. The entire rear roof of my van is covered in 1 inch of foam insulation. I also put foam insulation on all the windows in the rear, and made a sliding door between the front and rear of the van. The sun hits the metal skin of your van and turns it into an oven. The foam keeps most of that heat from going in the van. I always park in the sun, no matter how hot it is, I have all my windows closed and just my little swamp cooler running and I am nice and cool. Before the foam I was unable to spend anytime in the rear of my van even with the swamp cooler, it would get too hot.
I use the rtech foam they sell at home depot about 8 dollars for a 4 foot by 8 foot sheet of 1/2" foam. On the roof I have a 1" layer of foam. Originally I put a 1/2" layer but still felt heat coming through so I glued another layer on it. 1" inch did the job but you can add even more. 2 or 3 sheets is enough to cover the rear of the van. Use a hot glue gun to install it. Hot glue drys fast and is a very secure method to install. The original insulation that I removed was thin fiberglass like material that did not keep the heat out. There's no comparison between the 2 materials. I use a non contact IR thermometer that you can point at different sections to measure the heat. Anywhere you feel heat add more foam to it. Its easy to install just cut with a box cutter into small sections (eyeball it) no ruler need it, just make a rough estimate and glue it. Then cut another section. Similar to how they put heat tiles on the space shuttle, they use small sections. The reason to cut in small sections is the hotglue drys fast. Foam insulation is something that you can do that wont take long or is expensive and will give you good results.
For instance today the temperature in the front of the van was 136 degrees, in the back of the van the temperature was 96 degrees with just the swamp cooler running (without the swampcooler it was 106 degrees) .Where the swamp cooler was blowing it was very comfortable to me. Better than just a fan. And I have a small swamp cooler I hand built that uses less than 2 amps. I have 3 extra swamp coolers but never had to use more than one. And yes an extra swamp cooler running will lower the temperature more.
If I was in texas, I would just build a bigger swamp cooler, maybe with 2 x 9 inch car radiator fans (total power use both fans running 10 amps). I built one using just 1 car radiator fan but its actually overkill for my small van. I grew up in texas and all we ever use was a giant swamp cooler to stay cool, and it worked.
Foam insulation I think is the secret to keep cool. Enough foam and you can buy the smallest AC they make and just run it on low because it will get cold fast. Myself I wouldnt overlook a swamp cooler until I prove too myself it just wouldnt work and I would just build a bigger swamp cooler, a poweful fan can do wonders . The power savings are just too tremendous. 2 amps versus over 60+ amps for the smallest AC. I'm in california and the few times where the humidity was very high, the swamp cooler was better than nothing.
swampcooler I use