I realize the map might say 90 miles, but from my notes in my journal, it seems to be 78. We camped just short of the observatory, due to circumstances you can read in the entry. But it worked out perfectly anyway. You really don't want to camp behind the closed store at Kent; it's nasty. We ended up camping at the Lawrence Wood picnic area and it was great; that was 69 miles from Van Horn. But there are many places along the roadside, as Carla said, where you can pitch a tent. You can check my journal for details, and for more Southern Tier info too, at http://www.crazyguyonabike.com/doc/myfinecycle.
Most people we met along the ST actually went on US 90 from Van Horn through Marfa to Alpine. Just another thought. We really wanted to see the observatory.
It's a great ride either way, enjoy!