Bring snacks. AND plan ahead for simply another dining recommendaton to your question.
Have a late lunch/early dinner in Seattle before boarding. Bring fruit and breakfast pastries aboard for breakfast the next morning. Or, the 20 min stop in Whitefish at 7:15 AM might allow you to get breakfast goodies and/or lunch delivered while stopped (I think there are shops across the street from the station but not sure if they'd be open at that hour). Google to see if anyone will deliver to the station at Havre (20 min stop) at 12:45 PM for lunch and/or dinner. Ditto to see if someone will deliver dinner at Minot (20 min stop). Ditto to see if someone will deliver breakfast and/or lunch at St. Paul-Minneapolis (or if time allows, go into the station). After STPMN you're pretty much on your own to Chicago unless you Flex-dine. Again, bring snacks.
Note: You'd have to contact the vendors from the train a few minutes out via cell phone about the arrival time and your car location in the train or you could meet at the station. A late train could pose problems. You would also need to take a small YETI type container that could keep food either warm or cold. It would be great for someone to try to see if they can pull this off, even partially, and then post when/where they got food, what food was available, and the timing of the event. It might be a bit of a hassle, but if you pull it off you'd be the envy of everyone on the train. All of the above is subject to Covid-19 openings/closings of eateries.
Just another option. Good luck. Avoid Flex if you can at all cost!!!