I also vote for Grizzly RV Park in West Yellowstone. It is one of my very few favorite commercial campgrounds. Also, they will let you wash your RV right in your spot! Nice, big clean spots, and West Yellowstone has very good huckleberry ice cream.
I have a 32' Class C and barely fit into Fishing Bridge. I cannot imagine something as large as yours fitting in. Also, the end of one RV overhangs onto the site behind it, so that you will also have a back-end of something hanging over onto the rear of your site. (The sites alternate) If you try to park a car there, you will have absolutely NO space at all for even putting out a chair to sit on, and certainly no room for a meal outdoors. And if that isn't enough, many of the sewer outlets are underneath your RV, not next to it as one usually expects. I saw all sorts of bungled-together setups to hook up to sewer in that campground. SOme people had gone into the woods and found long sticks to use to prop up their sewer hose, which was hard to do when you had to crawl underneath to hook up.