Onion Valley campground is 9200 ft. 12k is roughly the elevation of Kearsarge pass, so maybe you're getting confused by that?

Regarding the steepness of the final 400 (which I have never done, but hope to some day soon): the best pictures I have seen of it in the snow are the ones in this thread, which sadly is about a tragic accident there a couple years ago: http://www.whitneyzone.com/wz/ubbthreads.php/ubb/showflat/Number/49640