The only time I've ever been waved at on the freeway while in the RV is when my outside storage door came open.
I usually wave at and or am waved to on side roads when something happens to warrant acknowledgement (e.g., letting someone pass me, someone letting me turn first, etc.).
And maybe that's the difference? With a motorcycle, you're used to being unnoticed by other drivers on the road - so a wave of "hey, I see you" is appreciated.
With an RV, you are always noticed. No one misses you driving down the road. So, acknowledging your existence isn't a big deal. Waving is, instead, reserved for letting you know there's something wrong.