I live in California. When I was shopping for a diesel pusher, various salesmen kept saying the 40 feet limit wasn't anything to worry about. So I walked into the local CHP office and asked three officers.
All three said there is definitely a limit, and it is 40' 0". Any vehicle longer than that is defined as "oversize" and requires a special driver's license, plus is limited to only designated main highways and just 1 mile off the highway to get fuel/food/lodging.
Do they go by motor home model number? Nope, they use a tape measure, bumper to bumper, and even 1" over 40 ft is "oversize" and triggers the requirements/limitations. Therefore, every single "model 40" motorhome I've ever seen is "oversize" in California.
CHP told me the highways length restriction is enforced for any "oversize" RV no matter what state it is from. All three officers said this violation is not something they go looking for, but when spotted they do not ignore it, and they have cited many drivers. The most common place to get busted seems to be wherever CHP is already looking at vehicles for other reasons.
The driver license requirement is not a trivial matter. When a California driver of an "oversize" RV lacks the proper Class B Non-Commercial license, CHP said the driver can not continue, and must call in a qualified driver to move the motor home! This license requires extra tests, and even a periodic medical certification. But I think the license requirement is not enforced against a non-California resident.
After I looked at the Caltrans map of which highways allow "oversize" vehicles -- not many -- I wasn't interested in driving all over my state illegally, or getting an expensive and annoying Class B license (medical exams?). So I bought a Monaco 38 model that was just a couple of inches under 39 ft. A Monaco company rep told me they make the model 38 specifically for California.
PS: All of this info is at Caltrans web site.