We had a 40' motorhome and found it to be no problem staying in national parks, national forest campgrounds, state parks, national wildlife refuges, county and city parks along with traveling down a gravel road to an awesome boondocking spot.
We've stayed in Rocky Mtn. Nat'l Park, Glacier, Yellowstone, Grand Teton, Zion, Bryce, Grand Canyon, Big Bend to name a few.
No - you won't fit in every site but there are sites that you will fit. Even with a 30' you won't fit in every site. Also, we traveled mostly west of the Mississippi but not solely. In the east there were many state parks and Corp of Engineer parks along with county parks that we've stayed.