Back in the early 70's when I started riding motorcycles on the street the standard wave was a fist in the air - I guess is was a take off on the "power to the people" thing going on back then. Over the years it's just become an acknowledgement of other riders. Some do the fist and first finger pointed to the ground, but most of us just give some indication that we noticed the other rider. When riding two-up it's the passenger's job to do the waving.
I also drive a Jeep Wrangler and there's also a wave acknowledgment between Wrangler drivers. This even happens in the city. Makes me wonder though how many of them have ever put their Wranglers into 4-wheel drive.
Some RVers do wave back, but most don't. However it's kind of a way to stay alert and observant. You notice another RVer and you give 'em a wave. I generally wave at RVers when I'm not driving in a city.
Waving to other vehicles was an old country tradition. Growing up in the sticks everyone always waved. There weren't any strangers, only neighbors you hadn't met yet. This still seems to be the case when you get out away from the larger communities. The further you get into the sticks, the more people will wave regardless of what they drive or what you happen to be driving.