We've driven the 160 route (through Pahrump)and on through Death Valley several times and it's definitely the way to go. If I remember correctly, it takes approximately 4 hours; of course, it depends on your driving style, and whether or not you are unlucky enough to have a slow-travelling RV in front of you. Your best bet is to get on the road as early in the morning as possible so that you're not crossing Death Valley in the heat of the day; same thing on your return trip home. When are you going up the Mountain?