No, it's not a myth. I can't say I've seen as much as some have claimed, but I've definitely seen my fair share while driving the semi. Heck, it was about 2 months ago (with my daughter in the pick-up), when I glanced over and saw some Mexican guy leaning way back, getting a BJ while driving down Gallatin Rd. It was on a weekend afternoon, very close to Rivergate Mall. Imagine all the traffic.
There is also a woman who's always on the CB around Nashville, and she's more than willing to flash the drivers. Crazy Lady is her CB handle. She leaves her home, around exit 112 on I-65, and travels to work somewhere around Nashville, near I-24E. The whole time she's on the CB and flashing truckers. She's been doing this on a regular basis for several years. She's a bit on the older side, but those huge breats are holding up surprisingly well.
So, it does happen.