A 30' Class A is like driving a car! You should be able to handle that anywhere... :)
If new to RV-ing with a Class A, you might want to stay off I-84, it's got a bad reputation for accidents on part of it (Deadman's Pass area) and has some twisty and steeper sections. Very busy route and is the only hwy in Oregon going east - west across the entire state. Google it if you're considering it. Wonderful scenic drive otherwise. Not a problem for me towing a 29' TT.
Hwy 26 is a pretty easy drive past Mt. Hood and very scenic - we were just there. One of my favorite routes. Grades aren't that bad and in the 6-7% range IIRC. Could take you farther south than you want at first tho. Watch for aggressive drivers and speeders east from Sandy.
Could go north to near Chehalis and take hwy 12 over to Yakima and then the I-90 over to Spokane and beyond. The I-90 route is many miles of open grassland and easy driving. Besides possibly hot in Aug., not a whole lot of scenery IMHO to get excited about. Can be very windy too. I would take hwy 26 east out of Portland over hwy 12.
Can't help east of the WA/ID border but have heard hwy 2 which runs all the way eastward near the Can/US border is a nice route.