Welcome to the forum. Since you're a "newbie", I decided to do a little investigating for you.
First of off, the campground doesn't necessarily restrict overall length to 40 feet. The website says the following, "Campground sites that will accommodate a maximum combined length of 40-feet or more are limited."
When you made your reservation, did you notify them of the length of your rig?
The website states, "Please keep in mind that most campsites in Yellowstone will not accommodate oversize units. If you arrive at the campground/RV park with equipment different than that for which you have reserved, we will not be able to accommodate you."
So, bottom line, it looks like you may be okay if you let them know up front how long your rig is.
