... back to the OP's question.... We liked using the 'Mountain Directory for Truckers and RVers'. It gave a lot of information on what to expect ahead. We didn't want to be just 'warned' of a steep grade when coming to it, we wanted to read all about to make any necessary rerouting decisions. Keep in mind that you'll have some of the same percentage grades on the western interstates as you will on the secondary highways. You'll see better scenery on the secondary routes.
We didn't like the 'Flattest Route' site... not very accurate.