Excellent and thoughtful information from "tatest". I expect there will be a variety of situations requiring many of the solutions presented here.
We would never try to use someone's private property without their permission. But I see nothing wrong with slipping into Public/business parking lots after hours as long as I'm leaving before they would be used. I'm curious from those of you who approve of such a practice, would you park in the back (hide) or right in the front (conspicuous)? To me, each has an advantage AND a disadvantage.
No doubt we will stay in a CG every few days for all the good reasons, but there will be times when it just makes sense to find a close spot for a few hours of down time. Almost to the primary destination, no campgrounds close, in between 2 minor attractions, late at night, etc.
We will probably never be to far from an interstate and the relative safety/legality of a truck stop/rest area, but this question was for those times when I am stuck without a "good option" and will have to take a chance.
And the answers, other than the "gestapo" ones, have been excellent. Thank you!