Hey CampsiteVactionlover.
First off, the map that is provided by rec.gov is way off. Campsite 17 is located in the upper or main part of the campground loop not the lower loop. The section where you see campsite 17 located, is the the lower campground loop and has sites 39 to 44, and yes, that is a dead end road.
Also note, when you look at campsite 31, there is a road that decends from the main Portal road down to the campground loop. That road is a closed road, used by the forest service or camphost etc, so don't view it as an access road.
Finally, when you pull into the main campsite loop, the road, after several campsites turns into a oneway loop.
Take a look at the link for views of Portal Campsites:
https://www.campsitephotos.com/campground/ca/whitney-portal/ (let the Portal Campsites load and select #17)
The camphosts are accommodating as long as your vehicle doesn't block the main loop road, however, I can't speak for them or the rules.