At 40' site availability will be severely resrictive in NPs & many older SPs. A CG may only have a couple sites capable of accomodating something that size & if the staff are not careful assigning sites to the appropriate sized unit or, worse, it is a first come first served & pick your own CG, you can very well find a class B occupying the only site that your 40'er could fit in meanwhile there are many empty sites that the B could have taken.
Saw that once. A Volkswagon Westfalia in a 60' long site. Talk about thoughtless!
The smaller the better is the rule but there is also the need to be big enough to be comphy. Somewhere there is a balance for everyone. We bought with CG site availability in mind. So far, 7 seasons of about 150 nights per season, very seldom making reservations, very seldom staying more than a few nights, we have been able to get into a site almost everytime. Our LOA is 35'