yep, we've done 50 in UT, camped at the Hickison Petroglyphs. Totally dry camping (no water) but a great stopping spot for one night.
How about Hwy 9 across the bottom of NM? I think all we saw was the occasional border patrol. Camped at Pancho Villa SP which was pretty good, particularly if you like sand. Cool museum at the park telling all about Villa's incursion into the US in 1916, the last foreign incursion onto US soil. We chased him but never caught him ...
NM 12/US 60 also seemed pretty darn lonely and desolate, at least until you come over the ridge and behold the Very Large Array. Really amazing if don't know it's there. 
And I think you probably meant 139 in *western* CO. Looks like a nice drive.