You might have problems in a few places but not many
- The Park Service campground at Grand Canyon South Rim enforces a strict 30' limit. If you want to stay at the Grand Canyon, you will either need to go to the concessionaire-run "Trailer Village" or stay outside the park, in Tusayan. The North Rim has a nominal 35' restriction, but the rangers there are more laid back, and will usually let you hunt for a space that you can fit.
- Many, many California state parks have limits well below 40'. Of all the states we've visited, only California has presented this issue. Many parks we simply can't do; a handful have a site or two where we fit.
- Organ Pipe Cactus National Monument actually measured us with a tape.
- Lots of Forest Service and other primitive campgrounds list limits lower than 40', but usually do not enforce this if you can find a way to fit.
- Many private parks charge extra by length.
- Death Valley NP restricts you to certain sites by length.
If you have anything attached to your coach that makes it longer than exactly 40', most of California will also be off limits. All that said, we have rarely been unable to go wherever we wanted. We've never been stopped and measured anywhere (except Organ Pipe). There is one road and one campground in Death Valley from which we were restricted, and a handful of roads with low clearances where we have to go around on an alternate route (such as the southern end of the Blue Ridge Parkway).