how do national parks measure the length of a 5th wheel?

We have found that the posted length of the site does NOT reflect the true status of the site. It does not matter, some are longer (by as much as 15 feet) to smaller. And the bullards are put in the weirdest places. I don't have an answer for your dilemma, experience is the best. Some times your own personal experience. Or the experience of another camper.