And I got it at fossil butte. At north end of park is a 17% grade loose gravel narrow road with two hairpins - good luck! But northern end of park boundary is well marked. Dark dark sky and unbelievably quiet.

Well maintained but also well traveled dirt road.  No camping signs are indeed posted, including not within 1 mile of the Rock.  Barely can hear 191, dark dark skies, next to an arch, within an agate collecting area, yes please!  But limited places to set up, free range cattle have fences up and gates closed.  If you don’t feel like roughing it, a $500 per night glamping site is on the other side of the Rock.

Easy gravel road after crossing I70. Would be hard to find the slabs at dusk. WIDE open no cover. Lights and noise from freeway. Good cell coverage. Great if just pulling off for a sleep.