Traveling from S ca. to Evensville ILL, with a 40foot fifth wheel, what would be the best route to take?

I would first check with mapquest. But then to be sure, I would check with the rv.net forum for 5th wheels and get some opinions from people who do the cross country thing often. Starting out by making your way to Interstate 40 will get you on your way. Then in Oklahoma City pick up Interstate 44 to St. Louis. Taking Interstates 270 then 255 on to south Hwy 3 will get you there. But my guess is that your concern may be grades. So the forum would be your best bet.