For the first part of the trip, locals, your friends and neighbors probably can give better advice than us - they can see your rig, and have driven the route(s) many times. Local news will also tell you a lot about pass conditions. There's also the State `Tripcheck` website.
Going east on 22/Santiam Pass is mostly uphill, peaking at 4600 ft, and only dropping to 3200/3500 for Sisters and Bend. After that 20 is relatively flat, but remote to Idaho. I84 in Idaho is fairly flat and straight, with a broad pass crossing into Utah.
