Polson to Coeur D’alene

We're taken 135 between 200 and I-90 several times and agree with paulj that the route is fine.
We used to spend the night at Quinn's Hot Springs on 135 to enjoy a great meal and the hot springs, but they no longer let RV's overnight.

135 between 200 and I90 is fairly flat, generally taking wide sweeping curves as it follows the river.

Another option is to stick with 200 all the way to Sandpoint. This follows a broad valley. In fact my impression is that it's straighter than I90, which has to climb to a pass on the state line, and other inside of Idaho. There aren't, though, many tourist facilities along 200.

Last time I drove this stretch of I90 was late on a September afternoon. Curving in and out of the setting sun and shadows was tiring, even on a freeway where I didn't have to worry about on coming traffic.

Yet another choice is US2 to Sandpoint.

looks to me that if time is an issue, go south on 93 to I90 West.

if time is not an issue then take the pretty route as mentioned above.

I haven't been through there but it looks like some pretty country and winds along the river. You might think about calling this place they are on 135 just north of St. Regis and see if semis and logging trucks use 135 from St Regis to 200. If they can you can
Big Sky Forest Products
(406) 649-2545

Your best bet is go south on Us 93 out of Polson, to I-90, then west directly into CDA.