We unhitch the van and park with the side door just under the trailer awning. I then drape a shade cloth from the end of the awning over the top of the van (propping it up off the van roof) and down the other side of the van. This creates a cool cave in the van and under the awning. If I am lucky the prevailing breeze will blow between the trailer and van.
The key is to keep the shade cloth off the roof; if it is touching the roof you still get solar heating of the van roof.
Since all of our "camping" is in the middle of open pastures you have to create your own shade.