I was specifically talking about assigned spaces. I've only camped in 4 different campgrounds in Yellowstone with reservations, but have yet been able to pick my spot. That's always been assigned at check-in. I've never been given a 40 ft spot in spite of being the only size listed on the reservation system that was available. So, it seems those assigning spaces have flexibility which is why saying your real length is important.
Last year at Canyon, a camper with a 30 ft fifth wheel stopped by and asked us about a very long space across from us. We said it had been empty. Never the less, they wouldn't assign him a space.
YMMV.