We easily fit in many national parks with our 40' motorhome towing the Jeep. Each park is different and there's no 'one' rule on RV size.
We fit in Yellowstone in Mammoth, Madison and Bridge Bay campgrounds - all without reservations. We fit in Grand Teton - Colter Bay and Gros Ventre campgrounds without reservations. We've stayed in Zion, Bryce, Grand Canyon, Big Bend. No, you won't fit in every campground or on any site but you'll have plenty of options.
Also, with a towed car most of the time, since it was so small, we could angle it onto the parking space. Not, so with a truck. No, you won't be able to park the car along the campground roads and the restroom area spaces are mainly for short-term parking for occupants or the handicapped.
We also fit in many, many national forest campgrounds, Corp of Engineers, state parks, county and city parks. We boondocked on public lands.
You'll hear comments by others who have a small RV who claim a big rig won't fit. That's only an opinion. That's not gospel.
Get the RV you will be comfortable in and don't dwell on it. If you don't fit there are plenty of places just outside the parks that you will. It's not the end of the world.