As others have said, it depends on the indivudual park and campsite. In my (quite limited) experience, very few parks have an overall limit that short. Such a limit might be due to a narrow, twisty access road. Many parks do have some sites that cannot accommodate a 30' motorhome and some that can. Here in Vermont, I find maybe half or fewer if the sites in the average state park comfortably fit my 32' class C. I have not had much trouble with finding suitable sites when I wish to, though it sometimes requires being flexible about destinations. Many of the Vermont state park campgrounds were developed by the CCC or in that era and so are laid out with tents in mind more than RVs.
Rather obviously, a larger unit will fit in fewer sites than a smaller one, and that's true at any size range. In practice, though, I don't think you'll see much difference there between 30 and 32 feet.
