Lancaster, CA To Yellowstone National Park

Get a copy of the Mountain Directory West (there is also one for the east). Use it to check your intended route to see what grades and elevations you will have and adjust your route if necessary. Avoid scenic routes. They are usually scenic because they involve mountain travel. On your way to Yellowstone spend a couple nights in Grand Tetons National Park. From there it is only 60 miles or so to Yellowstone. If you have time you might be able to slip in Zion and Bryce National Parks.