If you go before late June or after Labor Day, reservations are not required as the crowds haven't gotten bad yet. All you will need to do is plan to arrive at the campground early in the morning and wait for someone to leave. Just watch and you will see which ones are packing up. In July/August, you can still do this, but you will definitely be competing.
Another alternative would be Bridge Bay. While generators are allowed, tents are in a section by themselves and the rvs are over a slight knoll. No guarentees someone in the tent sections doesn't have a generator, but it would reduce the chance.
Unless I'm mistaken, there are also some back country sites which are relatively close to the parking lots. May pay to do a little research.
You could also disperse camp just outside of the park in the adjacent NF.
Just remember, Yellowstone is huge and driving speeds are slow. Unless you want to just pavement view the park, yhou will really need to stay at a couple of different campgrounds. Driving around a loop doesn't leave much time for getting far from the pavement.
