The answer is definitely I-10. Even though ElPaso, TX is 3800' elevation it's not like you're going from sea level to 3800' in one mile. :) You won't even notice the 'climb'.
That is true for any city elevation you look at. You have to take into account where the climb starts & at what elevation, then how long of a climb it is to get to the top elevation. Many folks get shakey seeing an elevation of say, 9,000'. But if you start the climb at 8,000' it's no big deal.
If you stay away from higher elevations you will be missing a lot of beauty of our country! We use the Mountain Directory all the time. Also, I just looked at the above link to Diesel Power magazine and it seems we've been on all those roads mentioned for the west.... We're not in the east much.
If you tell us where you're starting from and where you want to end, perhaps we could help a little more. Have a great trip - however!