As a newbie I remember reading a post of these forums. The poster said for state parks he would could comfortably subtract a few feet off his rig size and still fit in just fine, as he found state parks to be very conservative on their posted max size. OTOH, for private parks he added a few feet, opining that some private parks might try to cram a 32 footer into a 30 foot space.
I have not had a problem with private parks putting me in a space too small for my rig, but as I walk around FL state park campgrounds have seen many sites rated at 30 feet that would very easy work for a 34 foot motorhome (even considering access roads, turning radius, etc).
