thanks to gmaki for the info and your confirmation of it.
is it feasible to sleep in the car at the overflow parking near this 'hidden' campground if there isn't space in the campground or the weather isn't suitable for just throwing a sleeping bag down on a waterproof mat? (I don't own a tent)
When I was there last weekend a couple of my friends slept in their cars in the overflow parking lot from about 9:00 PM to 11:30 PM without incident. I don't know if it is strictly allowed but it doesn't seem like anyone is hassling people sleeping in their cars.
how far is it from the parking to the actual campsites?
The overflow parking lot sits right in between the backpacker campground and the walk in "secret" campground. I would say it is only about 20 yards UP to the backpacker campground, and 50 - 60 yards DOWN to the "secret" campground. It's also down a pretty faint and surprisingly difficult to follow trail (although you can see the campground the whole time so even if you lose the trail you will still eventually get there).
As I mentioned though your best bet is to leave the overflow parking, make a right as if you are heading down to the reserved campsite and park on the shoulder of the road right where the "road" to the campsites intersects the road out of Whitney Portal. (this road does not actually connect to the main road so don't think you can drive on it but you CAN walk on it). It is hard to explain but once you make it down into the campground the first time you can pick out a tree or a car as a landmark to your preferred parking spot and easily find it when you move your car. Once I got it down that is where I parked my car the rest of my stay and it made entering and exiting much easier. It also made packing up my campsite a thousand times easier
