Confronting a stranger with disgust or contempt for their behavior is not a course of action most would recommend. In an age when people have learned that even a bumpersticker can offend someone to the point that you risk finding your vehicle 'vandalized', getting 'in their face' could be extremely dangerous. People who are unstable mentally, or antisocial, or just garden variety jerks with a bad temper look just like everyone else, unfortunately. And the fact that you've just witnessed some antisocial behavior suggests that a confrontation would be unpleasant, at best.
On the other hand, Greg is right: we ought to be doing what we can to improve the negative image of truckers, so remaining silent doesn't feel right, either.
What I'd do, myself, is inform the carrier to whom the offender is affiliated of the behavior witnessed,(a phone call is sufficient, but in writing is better). I would also respectfully complain to the management of the truckstop, because they should be responsible for keeping their place of business clean, however they accomplish it.
It might feel futile, (and might be futile, sometimes), but maybe, with enough complaints voiced, the offenders can be persuaded by those to whom they are obliged to pay attention, that such behavior must stop.
Of course, if you think it's better to confront the trucker you saw throwing urine out the window on the spot, you can tell him what you think. (If you do, I'd be interested in hearing how it worked)