I replaced the front 1/3rd about 10 years ago with a EPDM paint after a tree limb tore a hole that became a balloon at 75 mph. Other than that I let the rain clean it.
I live in a dry area -most years- so it does not get washed very often. It is now 25 years old and has been outside the whole time. No leaks during the last "monsoon" season :)
I did repaint the sides a couple of years ago after the wife said it looked like a harlequin coat.