We owned a 36' Monaco Diplomat for 10 years. In those ten years we never really had an issue getting a campsite. Four years ago, we replaced our 36' with a true 40' coach. We still fit into the same spots we did with the 36', but we find that when we say 40', while talking to a campground, some can act like the world is coming to an end. We now say 39' long, seems to be less of an issue.
The biggest problme that many are currently facing is that the RV industry is BOOMING again and campsites are getting tough to get. If you want a site in a popular National Park, you better be ready to book a year out.
We've driven cross country twice in the past four years and never really had a hard time getting spots, except at busy parks or on holidays.