I also like the thought of a truck and/or boat gun. I always carry a firearm when I am in any of my numerous modes of transportation. There has never been a issue with the heat or other elements with reasonable care (don't forget to keep them clean and serviced).
The biggest concern is security. I make sure I remove the firearm anytime I am away from the vehicle for any significant period of time like overnight. Althought I lock my vehicles, they can still be broken into. Some might say just have an inexpensive firearm you do not care about being stolen. WRONG! There is no such thing since I am not worried about the value. I am worried about what happens once it is stolen or played with. Most of us would have trouble sleeping knowing we enabled some bad person to committ a crime with it or if a young under age person were injured or killed with a gun we did not properly secure.
Lets all keep the above as our foremost thought on a truck gun.