You really wouldn't want to use I-94 for any of this trip, but I-90 is going to be about the easiest and shortest way to get from anywhere in Washington to anywhere in Iowa. You'll take I-90 all the way to Sioux Falls, then cut into Iowa via I-29, and head to your destination from there.

Even starting from the Portland, Oregon area, it is only 50 miles longer to cut up to I-90 via US-395. The alternative is to use I-84 and I-80, but I-90 is a lower elevation route and often is a better choice in winter - although keeping an eye on the forecasts is your best bet to know which would be better during your times of travel.