It's a great route. We've taken it many times when heading west from the Rockies in our 30' class A. There is a hill going west over the Lolo Pass but it's no big deal and there's not much traffic to be a problem. We have camped at Powell Campground numerous times and then headed down along the river on a easy pace at 45 mph and in the morning there's very little traffic either way as you're 1 1/2 hours from Lolo and even more from the traffic heading east.
From Powell along the 12 into Oregon we then stay at the conjunction of I-84 and the 97 at Mary Hill Campground right on the Columbia River.
Great part of our trip.