Highway 189/40 to I-80

We are going south from I-80 to Spanish Fork, Utah and not sure if we should double back up I-15 to I-80 or take our motor home and toad up 189/40 to I-80 continuing east to South Dakota. Want to make sure not terrible steep with narrow roads, wife was none to happy making the climb out of Sheridan, Wy going to Yellowstone last year.