When we were new to RVing, I made maps, campground reservations, planned meals, etc. We were prepared for anything/everything.
Fast-forward a decade+. Last July, Ed announced that there were no forest fires in Glacier Nat'l Park (a Bucket List destination for me). I threw some clothes in a backpack; packed dog food (and some snacks for us) - and we left a few days later. (Granted, most of the essentials like shampoo/toothpaste, toilet paper, paper plates and cutlery; bedding and towels - are already in the van)
Bottom line - both styles work. Unless you want to stay at a popular State or National Park Campground (or have a very large RV) - most private campgrounds will find a space for you for the night if you call them the same day. Unless you are going to be waaaay out in the boondocks, if you need something, you'll always find a place to buy it. We planned a rough route, subject to last-minute changes depending on weather/fires, or discovering someplace too interesting to pass by :-)
The one thing I have that works great for these free-style trips: an app called "All Stays" (cost: $9.95). It will find you a place to spend the night, whether it's a fancy RV resort or a WalMart.