U.S. 2 from Kalispell heading East is a good road with fairly gentle grades. Don't think Mathilda will have a problem. As far as heading down to Great Falls, and Yellowstone, we have not been down that way, so can't help you there.
We have been to Yellowstone, but came in from the South, and left the park, heading East. Yellowstone and the Tetons are great, so plan to spend some time there if you can.
Have a great trip.