On those really hot days you need to start hydrating early before you start feeling thirsty. Drink water until you feel like your eyeballs are floating.
I hanged a water jug from the fire extinguisher every delivery I would drink water.
On super-hot i.e., heat warnings days water alone is not enough BUT is the number 1 thing to do.
You need to drink Gatorade /coconut water etc.… not much but enough water does not replenish electrolytes. Super-hot days you need to slow down take frequent breaks with the back of the truck pushing 120 out doors real feel around 108 or higher. Like I said cooling down your core temp is a great idea try a hose soak your body.
Pay attention to your body if it is shutting down stop working take shelter inside AC if possible or shade call the center. Once you have your kidneys shut down from the heat then you are prone to the condition more easily.