We've always kept a list of campgrounds (public) that we would return. From 16 years of full-timing I have a document on the computer that's 300+ pages, including boondocking spots. It includes lots of information about the campground. When we got to campgrounds, we'd walk the park and make note of the sites our 40' would fit. Our list never failed us in getting the best sites at times.
We volunteered at Rocky Mtn. Nat'l Park and assigned sites in the morning to reservations coming in matching a site to the RV size. We found there weren't many for big rigs. We offered to walk the sites and make a new chart of what size RV would fit into the sites. The rangers were amazed that we found many more big rig sites than on their old list and our new list was taking into account access and trees.