Ok, so this tip probably won't work anywhere but here in the U.S. and more than likely you won't be getting a hydro massage like in the image below. But you can get free showers just about anywhere in the United States and I will tell you how.
(Image Credit: By User:Aleksander Kaasik - Own work, CC BY-SA 4.0)
To get a free shower while traveling, simply stop at any truck stop. You can look up a list of truck stops in any area on Google Maps. Most truck stops offer showers. The best places are TA, Petro, Flying J, Pilot, and Loves. But there are many other truck stops than just those chains. The price is usually about $10 to $12.
Instead of paying for the shower, just walk over to the truck fuel islands and ask any trucker. Most drivers will have a extra showers on their frequent fueler card. Truck stops usually give a driver one free shower with each fill up. The showers last for up to 14 days on their card. Consequently they will have some left showers left over. Ask them if they could give you a free shower on their card. Most drivers are more than happy to help out.