If you're still working and pressed for time then definitely make reservations.
We were full-timers and retired so we were never on a schedule. We didn't make reservations even for our 40' motorhome and for many national and state parks - even along the Oregon coast in all seasons.
Many parks have campgrounds that don't even accept reservations such as Yellowstone, Grand Teton, Glacier, Bryce, etc. and also some state parks - even in Oregon. We also enjoy national forest campgrounds. Of those that do reservations, there are always 'no shows'. If it's a major tourist destination such as Yellowstone, we'd stay nearby the previous night and drive into the park early morning when folks are leaving and when the 'no show' sites become available.
Honestly, we've never been turned away because there was no room at the inn. I understand that some folks think they can't travel this way but you can. It just takes a different way to approach the situation. You'd never pull in a 4pm in a national or state park and expect to get a site. The same goes for pulling in on a Friday. If you can be flexible it's easily done.
