We reserve Federal campgrounds on Recreation.gov, which sometime shows both the total length of the driveway and the maximum length RV that can use it. That can be helpful in deciding which site to reserve in a particular campground.
Also, if the site driveway has a wheel stop at the rear, you can generally put your bumper several feet over the curb to add some length to the site. But we have sometimes had to move a picnic table to be able to do that.
What we have found with our 5th wheel compared to our previous travel trailer is that the height of the tree branches at a site is often more critical than the length. We have been in a number of state parks and Federal campgrounds where we have had to angle the trailer in the site to get it under the trees and then cannot open the awning. Ah, the trials and tribulations of having a modern RV!
My wife's parents had the first 5th wheel I ever saw (in 1973), which was basically a long, aluminum-sided pickup camper set on two axles. They would be amazed by the multiple slides, microwaves, LED HD-TVs, and electric fireplaces found in most modern 5th wheels!
New: 2016 Rockwood Signature Ultra Lite 8244WS 5th Wheel
2015 Ram 2500 CC-SB 6.7L Cummins turbodiesel 4x4
Sold: 2012 Rockwood Mini Lite 2109S