At last, you can trick chevrolet.com into giving you 2018 info by just replacing the 7 with an 8 in the URL:
http://www.chevrolet.com/byo-vc/client/en/US/chevrolet/tahoe/2018/tahoe/trim
http://www.chevrolet.com/byo-vc/client/en/US/chevrolet/tahoe/2018/tahoe/trim