Depending on where you are, temperatures can skyrocket in a truck / car. People had pets that died in their vehicles when they go someplace like Walmart to go shopping and leave them in the car with the windows up.
I think I read somewhere that temps can get as high as 130 - 140 degrees. Under those conditions I would only reluctantly leave laptops that have been MIL-SPEC tested inside a sealed vehicle. MIL-SPEC means military tested to be durable enough to operate in extreme environments including deserts. The ThinkPad T series is an example of a MIL-SPEC tested laptop.
If you cannot take the laptop with you and it must be left in the truck then I recommend you look for a solar powered fan that is installed in the window to keep...