Snoqualmie Pass on I-90 is a much "easier" drive. The grades are gentler, the curves less, and the road is 4-lane. With plenty of traffic.
Chinook Pass is a 2-lane roadway with some long pulls and some interesting curves and "edges". It is, however, a much more interesting drive than I-90, and is driven by RV's of all persuasions all the time.
Oh, and just west of Yakima, near Naches, you'll find fantastic fruit stands!