We have a 36' fifth wheel and we don't have problems finding campgrounds with sites that we fit into. That being said, there are sites within each campground that we WILL NOT fit into. If you frequent OLDER federal, COE or state parks, you may find that you won't fit. If the sites have been renovated in the last 10 or so years, you will more than likely find some that you will fit into.
I also think it has to do with being east or west of the Mississippi River. I think we may have larger sites here on the right coast. Just my $.02 worth.