My family and I are moving from Portland, OR to the Des Moines, IA area at the beginning of August.

My question is: What is the route with the least mountains/elevation change from the northwest to the midwest?

I am not concerned with attractions, entertainment, or scenery along the way.

We will be in a 16 ft moving truck, towing our car on a dolly behind us. (Front wheels on the dolly, back wheels on the road)

I'm debating I 84 to I 80 through OR, ID, UT, WY to get through the mountains.

OR

I 90 through WA, ID, MT, SD.

I want gas prices to be lower overall since all our belongings and our car will be pretty heavy, and I am not sure which way would be the best/easiest/cheapest.

HELP!!!