i don't know who does the measuring in the many, maybe most of the state parks we've been in over the past 31-seasons but I can tell you that the length-limits cited in the directories and web sites is often wrong or misleading. our most recent past MH was 35'/dual slides and we never had a problem finding a space other than on holiday weekends...which we tend to avoid anyway. we now have a 39' class A MH and while we don't use public parks as much as we used to those we have used have not presented a problem for us.
that's not to say that all public parks will accommodate us or that even a majority of parks in some area or region won't accommodate but I don't think the problem will be as widespread as you might think.
if you're thinking of using a specific park or parks why not call and speak to a human and ask about lengths?