Bowti, I called one park that had a 35' length limit and told the lady there I had a 40'. She said "did you say 35'?", and dumbly I said no 40'. and then she repeated "35'?". I finally got it and said yes 35', and she booked my site and said I should have no problem. Although, I have seen some sites in Florida state parks that would not accommodate a larger unit, most would handle a 30'. I was at Fort Desoto near ST Pete and could not get my rig into the site assigned, because of interfering trees. They allowed me to stay on their "overflow site".
Personally, I would call and tell them what you reserved and that you were thinking of buying a 30' unit and if that would be a problem for the site you reserved, and if it was, is there something they could do for you.