Some have pictures of the individual sites as well. A lot will depend on the layout of the site. I've been in some that the only reason the length was limited was because they had a cross-tie stopping the unit from backing any further. Some had trees or bushes right at the back of the site that prevented backing any deeper. Point is, you may be able to let the back end hang over some, but again, it will depend entirely on the site. We've seen some that the length listed was very optimistic and some that I couldn't figure out why they listed the length they did because there was plenty of room. We stayed at one last year that had the length listed as 30', but had plenty of room for our Pop-up opened (about 26') and my mega-cab truck without hanging into the road.
If the parks you are reserved in don't have pictures on the website, you might be able to talk with the rangers and get them to send you some via email.