The percentage of National Parks is not useful info. Neither is the percentage of sites in any of one of those parks into which you might fit. The only thing that matters, really, is whether a site big enough is actually available on the day(s) you want it. Also, the matter of whether you can reserve it or need to be there early enough in the morning and early enough in the week to snag it when it does become available.
As others have said, the National Parks were mostly built long before rigs that big existed. Since then, park budgets have been restricted so thoroughly by the penny-pinching crowd that renovations and upgrades have been pretty much impossible. So it is a bit of a miracle that any sites large enough to accommodate a big rig exist at all.
If you fancy spending much time camping in our National Parks, your best bet is tents. There are far more tent-only sites than RV compatible sites. Second best is a pop-up or small trailer.
You will find private RV parks much more accessible.