I learned the custom in the late '60s from my father, who did some 18-wheeler driving as a side business. He remarked at the time that it was a truckers' thing and that he didn't think very many non-truckers were aware of the practice. I think it's quite possible for the same to be true today: most non-truckers don't know, and among those who do know many may not care.
I usually flash for the trucks who pass (and I get about 50% response rate), but I don't usually flash for RVers because I figure they are much less likely to expect or want it. Hardly anyone (trucker, RVer or otherwise) ever flashes for me when I pass, not that I care a whit. People can do what they wish, and I will do what I wish.