You have to keep in mind most National and State Campgrounds have been around since the 40's and 50's or longer. They were designed back when most people were tent camping and RVs were the Canned Ham type travel trailers which usually were under 20 feet in length. The roads and campsites were built to accommodate this era of campers. The parking spots at the campsites may accommodate much larger RVs but the twists, turns and trees of the roads in the campground may not.
What we have found is at some campgrounds there will be sites accessible by larger RVS but it is hit and miss whether these sites are available and if the staff will allow you to enter with a larger than maximum length RV. We've actually have had rangers escort us the wrong way on the roads in order to get into a spot that would accommodate our 38ft 5th wheel. Of course this will only happen on slow days when the campground is lightly occupied.
Most of the time, instead of trying to find a National or State Campground that is big rig friendly, we choose to stay in private RV Parks located near the National/State park. We drive in daily to see the sights and then come back to our RV with full hookups and all the amenities the RV park has to offer. In our opinion-It makes for a less stressful experience.