While road construction can make a stretch of road slow and rough, I don't expect routes in this part of the country to change that much over the years.
I drove this route nearly 20 years ago. I generally followed a diagonal on the paper map. US89 north of US12 crosses a mountain range (colored as National Forest) that was pleasant, but might not be optimal for a RV seeking flat land. But most of the route is wide open high prairie.
US89 Great Falls to Browning was fine, generally flat and straight. However after Browning it gets into the foothills and becomes more windy and up/down. At the end of full day that was tiring, even in an easy driving SUV.
Other posters have recommended Duck Lake Rd, MT 464, as flatter and straighter.
That said, I vaguely recall posts talking about road work on 89, so it might now be a better drive.
Google Maps streetview gives good details on roads, though I see all the images in this part of Montana date from 2008. But the sat images are new. Also look at Terrain mode to get idea of the grades.
Google Maps 'live-traffic' may also flag (major) construction. State DOT also has construction information.
