Many roads leading to eastern Sierra trailheads have signs that say "CAMP ONLY IN DESIGNATED CAMPGROUNDS" or something to that effect.
Webster defines camping as making or occupying a camp. Camps involve tents, cabins, and the like. According to Webster, discretely sleeping in your car at a trailhead is not camping.
However, the USFS has seen fit to redefine the word "camp." If you read the fine print on the signs around trailheads and campgrounds, you may discover that any outdoor overnight occupancy of USFS land counts as camping. Sleeping in your car is therefore illegal at most trailheads.
I've been sleeping at trailheads since 1987. I initially slept in a camper. When I was temporarily out of work, I drove a Honda Civic to save gas, and I slept in a bivy sack. It worked so well that I quit driving the truck unless I needed 4WD. I now have a comfortable bed in a car that appears much too small for sleeping. I've slept at trailheads hundreds of times.
The only time I've ever been harassed was when a campground host extorted $10 from my wife while I was climbing Broken Finger Peak in 1993. We were parked in a space marked "HIKER PARKING" and he charged us for a campsite we never occupied. After months of phone calls and letters pointing out that this wasn't a "fee area," and that charging a fee there was illegal, we got our $10 back.
Bottom line: Be discrete. Don't set up a tent or do anything else that suggests that you plan to be at the trailhead for more than a few hours sleep. Don't make a mess or cause a lot of noise. In short, don't give anybody a reason to complain and spoil it for all of us.