I've been riding for ~20 years, and I... pretty much wave at any recreational vehicle on the roads with me. RVs aren't recreational vehicles, they're pylons. But bicyclists in the middle of nowhere, vintage cars on a putt, any bike or trike... I wave. The vast majority wave back. Only time I don't is when I'm in a situation when I need hands on the controls, and I really think the folks to don't wave back are mostly in that situation.
But I think there's a language of waves.
( 1 ) In the rain, or <= 40 degress, a wave always means "Yeah, buddy, this sounded like a good idea to me too."
( 2 ) In summer in seattle, waving like a sugared-up 8 year old in a community parade, on a float, on _crack_: "The sun! The beautiful sun! We're so lucky! We win! Isn't this great!?"
( 3 ) The summer, anywhere else, low wave "Man, this is great, innit?"
( 4 ) Middle of f'in nowhere, I'll wave at any other vehicle, period, high wave, "So nice to see someone else! I'm sure you have some good reason to be here too! I'm fairly certain that if I was on the side of the road in a ditch, you'd tell someone in another 2 hours and they'd come running. That's great! I'll do the same for you!"