If you want to avoid mountains go further north, cross the Mississippi in Iowa (avoiding St Louis) and west from there missing most mountainous areas all together until northern WY or western MT. If you go thru the Black Hills you'll see a few big hills.
This involves a lot of Interstate hopping in the midsection of the country--I75-I64->I-65-I-74 to Iowa. But it's flat. You can get on I-80 and stay there or hop up to I-90 which runs you right thru MT. We've taken that route a number of times whenever we are going to the northern tier.