How safe is it to leave your camp unoccupied for several hours when road camping in the desert back country?
Perfectly safe, since the NPS will personally assure that no harm comes to you or your stuff.

If you pay extra (rates are negotiable) they will assign a ranger to guard your equipment while you frolick in the desert.

Seriously, your stuff will most likely be fine as long as you don't leave the diamond ring or camera sitting out.