OK this is just my experience. Anything over 35 ft is going to have a problem in state and federal sites. With States of course it will vary state to state depending on when their CG were built or updated. Almost all Federal Parks for certain and to some extent Forest Service CG were all built in the 40's thru the 60's, People did not have huge RVs and the parks were built with smaller campers in mind. So you may find an occasional site that will take a 40 footer, but I wouldn't depend on it. And even if you do it may be one or two sites in a 100 site CG and they are in high demand.
COE CG seem to be the exception they have huge long pads for campers, 75 to 100 feet in some cases.
Anyway that's why we have a 34 footer.
Of course commercial CG are plentiful. And they can take large campers in many cases. But IMO you lose a great deal by camping outside major parks. First the parks are very big and it can take 90 minutes or more to drive into the park every day and the same to get out. Secondly you miss the Ranger programs at the CG