Hwy 20 from Mt Vernon to Omak?

We loved the drive on 20 from Omak to Mount Vernon with our 40' MH but yes, you'll have curves and grades. The biggest in Rainy Pass west of Winthrop. Westbound descent is 4 mi of 6-7%; eastbound 7 miles of 7%. Just depends on your feel for drives like this.

Recommend you take 2-3 days to do the "Cascade Loop". Take US2 Everett to Wenatchee, US 97 north through Chelan and up WA153 through the Methow Valley to Winthrop. Take WA20 westbound from Winthrop to Burlington (OR you can take the WA530 cutoff from 20 at Rockport through Darrington and Arlington to WA9/I-5).

At many spots the Cascade Loop is a stunningly beautiful drive.

Sedro Wooley to Newhalem is worst part for larger rigs. It is curvy and lacks shoulders. Main portion through North Cascades is good. Yes, there is good downhill at Liberty Bell Mountain.
Numerous pull offs and view points.
Make sure you have plenty of fuel. Eastbound last fuel is near Marblemount.
Much better then using I-90, shorter then I-90 and US 97 to Omak.
Next option would be to take US 2 and then return via WA20 or reverse.

Beautiful drive. Just make sure your braking systems are in good order for the big drop at the end.

It is a good two lane although twisty in spots so don't try to rush. Beautiful trip although the east end through Methow was severely burned in last year's forest fire so be prepared for extensive devastation.