You will not have any trouble from Deming to Alamagordo as far as elevation changes go. If you are unlucky or lucky, depending on your point of view, there may be a 1 or so hour delay while White Sands Missile Range fires a missile across the highway. That is a rare event though. Do not try to drive through Las Cruces on US 70, use I-10 to I-25 north to US 70. 70 is 4 lane all the way to Alamo and as mentioned is flat after the decent from the top of San Augustin (5700ft).