A couple of years ago I rode along with a friend on
this ferry trip. We crossed no high terrain and much of the time were below 1,000 AGL but did go high for a while in Oregon to clear weather.
We had to go to El Paso, but you could go from Saint George toward Albuquerque then Dallas.
East of the Cascades, we were flying pretty high.
It wasn't winter but it was wintry. We followed a basin for a long way in the basin and range country in Nevada to avoid having to go high there.
Snow Canyon is just north of Saint George, Utah.
After Saint George, there is still the Pariah River, Grand Canyon and Monument Valley. This route may not be the exactly lowest and easiest, but it wasn't bad, as we were able to fly it in fairly bad weather, and it was pretty much on the scenic side. I can't say we flew over a lot of populated areas though.