Before we bought our fiver I looked at a lot of western national park sites and it seemed as tho there was a cutoff around 35 ft. That is by no means a firm rule, Chiricahua NM for example limits RV's to the 20 so odd ft length and Yosemite only has a few sites for 35 ft RVs. A lot of National Forest CG only take shorter RVs often because of the access roads. State parks will vary all over the place.
A lot of this depends on the age of the CG, if they were built back in the 40's or 50's then they were not designed with long RVs in mind.