I don't think most people know that it's NOT OK to sleep in their vehicles at trailheads. Even if there are signs posted 'No Camping' I think most people interpret that to mean they can't setup a tent, and if they're sleeping in their car/truck then so what? They aren't taking up any more room, imposing on anyone, etc, so no harm no foul.
My hunch is that 10-20% of the vehicles you see at a trailhead early in the AM has a person(s) sleeping in them, especially in the warmer weather. If the windows are misty - bingo!
I think the chances of being rousted by a USFS ranger for sleeping in your vehicle are rather low. THe USFS is shedding staff rapidly, and my hunch is that patroling the parking lots for parking pass compliance, 'illegal' camping, etc is a low priority.