How long is too long to travel In a fifth wheel.

I have a 40' 5th wheel, and while it has cut me out of some of the older parks and some state parks that were designed for 19' trailers, I haven't had an issue. One time when I tried to register at Bayou Signette State Park in Louisiana Near New Orleans, the lady asked me the length, and I told her 40', and she said "35'?", and I said no 40', and she said "I must have misheard you, because our length limit is 35", and I said "yes maam, it's 35'". I did not have a problem fitting into the space.