Yellowstone doesn't measure your RV. We've stayed in a 35' site with our 40' motorhome. The 35' is a 'recommendation'. If you reserve it then it's yours whether you fit in it or not. That's why we don't reserve. We drive in and check the sites ourselves in an unreservable campground or in a reservable campground that just had a cancellation. We've been to Yellowstone many times and never had a problem getting a site.
We also volunteered in Rocky Mountain Nat'l Park and one of our 'duties' was to measure all the campsites in the campground that we were stationed and to make a chart of what size of RV would fit in the sites. This was for the purpose of assigning sites when folks come in.
Every site is different, especially in public parks. It depends on tree or boulder positioning. Also, if it has a rear concrete barrier on the parking pad, can you back in further so your tires go to that barrier rather than the just the rear of the RV? Just by being able to do that can increase the parking space by 5' or so.
So your specific question of using a 36' in a 35' space. Yes, you probably can. One foot isn't a big deal. Also, keep in mind that if you tow a car you'll need to fit it on the site, too. The majority of time we angle our Jeep in front of the motorhome and it fits in fine. Sometimes you will have room beside your RV parking space but not often in public parks.
Good luck. I'd advise you to get a RV you feel comfortable with and you'll also find sites to fit even if it's nearby to where you really want to be.
Full-Timed for 16 Years
.... Back in S&B Again
Traveled 8 yr in a 40' 2004 Newmar Dutch Star Motorhome
& 8 yr in a 33' Travel Supreme 5th Wheel