We drove from Leavenworth to Everett in late April, and it was not bad. Still skiing at Snoqualmie Pass ski area, so it's pretty high. It was a late Spring, so maybe not normal?
The road climbs pretty slow, so not a big problem. Plenty of passing lanes also, so take your time.
Enjoy Leavenworth. Lots of great murals and walking around you will be sure to find a fine meal if it's lunch time.
Ellensburg to Leavenworth will be a good pull also, but plenty doable.
Happy Glamping, Norman & Elna. 2008 Winnebago Adventurer 38J, W24, dozens of small thirsty ponies. Retired after 40 years wrenching on trucks! 2010 Ford Ranger toad with bicycles or KLR 650 in the back. Easy to spot an RVer, they always walk around with a screwdriver or wrench in one hand!