Both Hwy 89 & Hwy 20 are excellent with any size RV.
The westbound descent on Hwy 20: you'll have about 2 mi. of 8% then 2 mi of 4% then 3 mi of 5-6%. It's a good two lane with 45 mph curves.
As with all downhills, get into lower gear before beginning the descent; not in the midst of it. Then you won't have to use your brakes much.
Have a good trip!
Full-timed for 16 Years . . .
Traveled 8 yr in a 2004 Newmar Dutch Star 40' Diesel
& 8 yr in a 33' Travel Supreme 5th wheel