As stated, depending on the size of your 5th wheel it's advisable not to take it on Hwy 1. 101 is very doable with any size.
We have gotten sites in the Oregon State Parks mid-summer without reservations. I really don't think you'll have problems in April/May without reservations. Reservations really ties you down and puts you on a strict schedule. We dislike them and have no problems finding a place to stay.
Once you're on the Olympic Peninsula, plan a stop at Port Townsend. It's a neat little town and you can do a lot of day trips from there including via ferry - to Seattle's waterfront and to Victoria, B.C. You can easily tour parts of Olympic Nat'l Park from there. You could also put your RV on the Port Townsend ferry which will take you to the area north of Seattle - Anacortes - for the continuance of your trip.