I brought a van last year (and am bringing one this year), here are some tips:
-as mentioned before put tinfoil/those windshield foil protectors/hell even towels over the windows to block the sun as much as possible. While during the day it will never be "cool" inside the van it helps drop the temperature a couple degrees.
-As far as sleeping goes: at night you'll be fine (as it can get down right cold there at night), just open the doors up for a little bit during the evening hours to let some heat out. During the day just leave whichever doors aren't facing the sun open to get some air.
-A cooler is fine inside the van. The way I worked it was I had two coolers. One large one with all the food/BEER/other stuff and another that was JUST ice. The one that was just ice i didn't open until the the ice had melted in the big one. Then I just added as I needed. The ice lasted me from Sunday night to Saturday afternoon like that.
-Spend a little time and money on making a shade structure off the van... trust me it's nice and not that hard to make. (especially if you're not camp with anyone with shade)
here's the shade I made:
Any more questions let me know!!