Question on determining campsite size?

We always give a total length and then a foot or two. It also depends on the park. If it's a campground with smaller sites like a national park, then I often have to keep it to its true length or can go a little shorter since I have about 10' of overhang in the back and as long as there is nothing behind the site stopping me from backing the wheels all the way to the edge of the parkign pad. But if I have an rv that is 30' long, I don't want to tell them 30'. Becuase they may stick me on a site that is only 30' long. Last thing I want is trucks and motorhomes driving just a foot or so from the front of my RV. Now I don't tell them I need 50' if I need 30'. But I wouldn't hesitate asking for 35'. I don't know what the person is thinking or what they will do. And will pretty much suck if I get squeezed in on a site.