I've got a MaxxAir MaxxFan vent installed in the cabover. It can be open 24-7, driving or parked, on or off, since it's pretty near rain-proof. Since the cab windows are shaded by the cabover, I leave those windows down about 1/2". This gives me a slight 'chimney effect', since the hot air tends to rise thru the vent, and draw cool air in at the windows.
I'm currently trying to jump thru the city's freakin' absurd bunch of permitting hoops to put up a metal carport for it.
Jim, "Be obscure clearly."