So,

After the higher campgrounds close along the 120 in Yosemite, do the rangers still look out for people slumbering in their vehicles or near the trail-heads? Are the USFS campgrounds the best option then?

You'll probably want to stay outside of the park. Right around when, if not exactly when, the campground and store close they start to forbid any kind of overnight parking along 120. Presumably so it's easier to close down 120 in the event of poor weather. Maybe there's a small gap of time in there, when the facilities are closed but overnight parking is still allowed.
Hope that helps. It occurs to me that maybe everything is closed already? Haven't been up there in a couple of weeks...
Jed