One would hope you would slide it under the seat or....behind, or something, to be less tempting to thieves.
Severe rapid temp changes would hurt it more than just getting to 120 ish F or whatever during the day in the truck.
For example going from the hot truck to a cold air conditioned environment would possibly cause condensation.
Since a lot of laptop operating temps for laptops seem to be 80 C plus, I can't see the truck getting that hot, so it shouldn't hurt it especially.
Moisture from whatever source might be more of a concern. ( rainy days, etc.)