I always check the campground website to see the length restrictions. We have a 38 foot 5th wheel and if the site shows 34 or less I will look for another campground (public or private) near where we want to be. Great thing about National/ State Parks there are usually private campgrounds nearby. Private campgrounds tend to be setup to handle big rigs.
If the website shows 35 feet I will call to ask if a 38 footer would work. Most of the time it will. Occaisionally we have had to drive the wrong way (with permision) around a campground loop to get to the site. Most of the restrictions have to do with narrow roads, tight turns or obstacles rather than the actual site size.
For Sequioa National Park we stayed outside at a really beautiful RV park next to a river. None of the sites inside the park would have matched the site we stayed in, I would rank it a 9 out of 10.
My advice is to get the size and floorplan you are comfortable with, there's always a solution for finding a spot as long as you are willing to drive a short distance to get to the National/State park to sight see.
