Along the coast where the redwoods are found you will have the best chance for sunshine and avoiding coastal marine layer gloom in April and early May. Plan on temps in the mid 50's except around the Willits to Rio Dell section where it can be cooler at night and warmer during the days.
Most scenic route is from Pt Reyes up Hwy 1 past Mendocino and then cutting inland through Anderson Valley and over to Hwy 101 near Leggett and continuing on Hwy 101 up past Orick and to Prairie Creek State Park with its two herds of elk. If your RV is short enough the campground at Gold Bluff Beach is a great place to stay. This is as wild a beach as you will find anywhere.
Don't miss Mendocino and Ferndale on your travels.