The good thing about most public campgrounds they are located in popular tourist destinations. That means there will be at least one private campground nearby. Most private campgrounds are big rig friendly. So eventhough the public campground may not allow longer than 35ft RVs you can find something at a nearby private campground and drive in to see the sights.
With our 38ft 5th wheel, we stayed outside of Kings Canyon and Sequoia National Parks at a great Private Campground (Sequoia RV Ranch) which was a short drive to the entrance gate to go into the parks to see the sights. We had a great spot overlooking the Kaweah River.