It took me trial and error to deduce what that button did; I'm glad to have others confirm it.
If your Tiff has other controls similar to mine ...
AND if your experience with it is similar to me ...
then now is the time to ensure you know how to operate the windshield wipers. Within 30 min. of our taking possession of our Phaeton it started raining and we couldn't figure out how to operate them. We wound up pushing the (wash) button to get a single swipe. Turns out my smaller circular button rotates much like clock hands. The outer/ larger circular button rotates to set the delay of swipes.
One other overlooked tip about the wipers - once we operated them properly (of course the window was now dry) the rubber just rolled up and peeled off the blade; so check them also, before they're needed.
We still haven't mastered using the installed radio (and setting the clock); its not of much use to us over the road noise and once camped out we prefer peace and quiet.
2010 Tiffin Phaeton 40QTH
2014 Honda CR-V FWD Ex-L
LA (Lower Alabama)