Is it really too far to walk to the bathroom at a rest area?

I don't know. I've seen enough out there not to be surprised by anything anyone does. I suspect you may be right about the piss bottles - I've never seen nor met anyone who does or has admitted to doing that (but I wouldn't be surprised that several people I know have done it). But, as for pissing in parking lots, I've seen people from all walks of life do that and think nothing of it.