Yellowstone (and much of the National Park Service) campgrounds are "sized differently" than many public and private campgrounds.
In Yellowstone, the campsites are sized "in combination" with the tow vehicle. That means a 40' site will accommodate a RV and tow vehicle length of 40'. Here's the comment cut from the Madison Campground website:
"When calling to make a reservation be prepared to give the size of your tent (in feet) or the combined length of your RV and any other vehicles or towed vehicles. (e.g.
the length of your truck in addition to the length of your trailer when fully open.) Campground rates are per night, for up to six people or one family (parents with dependent children) and do not include taxes and utility fee.
Campground sites that will accommodate a maximum combined length of 40-feet or more are limited."
http://www.yellowstonenationalparklo...on-campground/
We drove through Madison campground last year while in Yellowstone, but did not camp there. We stayed at Fishing Bridge. Many of the sites at Madison are "pull through loops" as well as the conventional "back in" spaces. There is ample parking at Madison campground, so while there might be an "issue" with your combined rig length, your RV will fit on a 40' site and there is additional parking at the campground, so at the very worst, you may wind up parking a couple hundred feet from your RV, but at least you won't be turned away.