Today I was looking at Reserve America for a site at a SP in Maryland. Reserve America showed every single site with electric hookup as 30'. My 5w is 32' actual nose to tail. So I thought I would not be able to use this campground.
But just to make sure, I called the campground and the ranger gave me a list of seven sites (out of 26) that would accommodate a 32' 5w. He also assured me that it would be no problem maneuvering around the loop, and that no one would come by with a measuring stick. So I went to RA, lied about the length of my rig and reserved the site.
The point is that RA, and other web sites, depend on the park manager to provide the length information, so the information is only as good as the park manager makes it. As said previously, there is no "standard" way to measure a site. If you are not sure, call the park.
