Each national park has sites for varying lengths. Our 40' motorhome fit easily in Glacier, Yellowstone, Grand Teton, Zion, Bryce and others. We also made our stays in national forest campgrounds, Corp of Engineers, state parks, county and city parks. You won't fit in every park with a big rig and you'll learn to do research beforehand but neither will a 30' fit in every park. Get an RV that you'd be comfortable in. There are always options outside the parks.