Our 40' motorhome and Jeep fit in national parks and we didn't use reservations. Some campgrounds don't even accept reservations. We've gotten reservation sites just by driving up and snagged a cancellation or no show.
We fit in Glacier, Grand Teton, Yellowstone, Rocky Mtn. Zion, Bryce, Arches, Canyonlands, Big Bend and many, many national forest campgrounds and Corp of Engineers. Again... that was with a 40' motorhome and Jeep.
No... you won't fit in every site but neither will a 30'. Get the RV that you will be comfortable living in. There will always be a place for you!
