When I first saw your post I thought you were referring to something entirely non-RV related and I was relieved to see you were playing it straight.
I used to drive a truck and the courtesy of flashing your lights is very much appreciated. Some will acknowledge your courtesy and some will not. Some will do the same for you and some will not. If there is one group that I have found that doesn't seem to ever either flash for us to pull in or acknowledge our courtesy it is the truckers from Quebec. Not sure why they don't acknowledge, but they don't.
I've always taken the attitude that courtesy is contagious, just like stupidity is also contagious. Courtesy makes life more fun.