WA21 is mostly valley driving, with minimal divides between river basins. The side road to Midway is also flat, though less traveled. It follows the Kettle River as it makes a loop in to the States.
But getting to 21 will involve grades of some sort.
My guess is that WA20 from US97 will be the easiest, though it's been some time since I've driven it.
US2 east from Lincoln Rock has a significant grade to the Waterville Plateau. Then there are grades back down to the Columbia River.
WA 155 from Coulee City north isn't as steep, but still climbs before dropping down to US97. There are a couple paved local roads from 155 to 21 but they do cross a ridge. Check them GoogleMaps Streetview.
You can also catch 21 at Wilbur (US2), and take the ferry across the Columbia. That's a pretty crossing, and nice campground at the reservoir. The drop at the end is brutal - but I'm sure lots of big RVs take it.
There's a new ferry on 21. It was assembled from oversize pieces near the Grand Coulee Dam.