On hot days, Ill stop at the nearby convenience store and buy a gallon jug of store brand water before heading out.
It will be empty upon my return- And because of the heat, I won't need to use a bathroom anywhere along the route either. I just sweat it all out. I've learned the best thing to do with an LLV is to close the door leading to the back, open both windows all the way up and set the fan so that it blows across the cab. Sure, the mail in the back gets pretty warm, but being the evil fascist rethug that I am, I'm really not that concerned about how comfortable the bills and catalogs are.
Been there, done that.
When I worked for a delivery company, all our trucks were 14' aluminum bodied Grummons with Chevy 1-ton chassis. I would carry a 2 gallon water cooler with me all the time.
One very hot day, I was running a route in Charleston, WV, and I thought I had stepped in some chewing gum somewhere, because my right foot was sticking to the metal floor. Turns out, the sole of my shoe was melting! I also emptied the 2 gallon cooler, and had to stop for a bag of ice and a gallon of water.
The next day, I upgraded to a 3 gallon Gott cooler, with the spigot on it.