When summertime heat takes hold, the interior temperatures of vehicles skyrocket. The glass and sometimes dark colours of vehicles add to the effect, and the temperature inside a parked vehicle is often much higher than the surrounding air.
As you get into the summer season, it is a good idea to do an inventory of your cab. What you want to look for are things that are negatively affected by heat. Chocolate bars? They become chocolate sauce. Water bottle in the sun? Might get hot enough to make instant coffee.
Those who live in their trucks while out on the road often move in and store odds and ends throughout the vehicle. Make sure what you have in your cab and sleeper is appropriate for the season.