Just returned from Williams, AZ for the weekend.
66 thru Flagstaff isn't much these days. It's the main street thru town but remnants of yesteryear are few. Museum Club is fun old bar and grill that has existed since the Route 66 days mostly unchanged.
Williams is a ton of fun. The town still has it's 50's vibe complete with neon signs and several diners, steakhouses etc. Lots to do around the area such as Bearizona drive thru animal park.
66 from Ash Fork to Kingman is one of the longest remaining stretches left. Seligman is another fun town to stop at. Grand Canyon Caverns and nearby Keepers of the Wild animal sanctuary are worth stopping.
66 from Kingman thru Oatman has poorly maintained stretches and tight turns. I have not tried with an RV but is a fun trip in a car.