Sand:
-Kermit dunes
You have the option of paying for a campsite and optional RV hookups (http://www.kermitdunes.com/) or camping at the free parking areas with no hookups or utilities.
There's various small dune fields all over the west Texas area, some publicly accessible, others require permission (that means you have the dunes all to yourself...all of it virgin sand).

-El Paso has some dunes, not big but it's closer to the Glamis scene, mostly lots of baja/desert XC trails out there.
-New Mexico has some small dunes as well.

Not sand:
Pecos river trails (lots of open space, bring some spare fuel on your rack), 3 moderately sized salt flats south of me, one really big one west of me.
Outback adventure track in Stanton TX (http://outbackadventuretrack.webs.com/)

About the only thing worth taking a trip out here for is the Kermit dunes and maybe the salt flats near Dell City for speed runs.
Not worth taking a trip specifically for anything else unless you live in TX/NM...