What is the single flattest route to take from Chicago to L.A. if you're dragging a trailer behind your car, and the car doesn't like hills all too much in that situation, and you want to take a more southern route (given the weather as of late)? And the more direct route, the better.
I need the single best interstate (or interstates) to get through with minimal strain on the car. Just to give you something to compare to, it didn't like Pennsylvania and Interstate 80. I was filling her up almost every hour and a half.