Yep, definitely depends on the campground. We couldn't get in to quite a few National Parks because they were "full", even with a whole bunch of empty grass. Which might mean another hours ride, with all the no trespassing signs everywhere else. Or we'd have to pay for a full sized RV site because the concessionaire needs to make their cut.
State parks are generally better with hiker/bikers, although we did go to one hiker/biker/kayaker where every site was full of car campers and we had to wait until after 7pm to get the disabled site. Nice try, but don't put a hiker/biker campsite next to a car park would be my recommendation.