We try to stay away from high demand campgrounds in the peak season so can't address your Yellowstone problem but like "tatest" wrote you need to adapt to the problem. We did Yellowstone in early June, no problem with camping but it was cold at night and the upper road had just opened. We almost never make reservations and if we do, it is mid afternoon when we know where we plan to be. When we could not find a campground with space, we have driven much further than planned, stayed in motels a couple of times, stayed in some very questionable campgrounds, rest areas, Walmart, etc. It helps to be able to live self contained for a day or so when necessary, we can go about a week if needed.