Depending where you are going, if you are reserving a site online, you may need to put in a smaller site size than the total length of your rv. For instance if you have a 35' long rv and the largest site is 30', then you will probably fit as long as there is no rear obstruction. You will probably have about 10' of overhang behind your back wheels. Thus keeping you on the pavement (if there is any). You can always back up to the edge if possible and have enough room.
But as others mentioned, they do sometimes post limits on size. Bt I don't think they are gonna get a ruler out.