If you're ever at the airport, those restrooms are real busy, but surprisingly clean most of the time, especially if you go in right after the cleaning crew makes it's rounds.
Most truckstops smell like doodie crawled up into your nose and built a nest, but they are clean sometimes; Walmart is on this tier too. Restaurants are mostly bad. They usually don't have dedicated cleaning staff and nobody wants to do it.
Public rest stops and park visitors centers are next. These are all governmeent employees and there's only the bare minimum of them, so they don't get cleaned often. Maybe a couple of times a day.
Last is regular gas stations. These are the places where you can catch a disease from sitting on the toilet and people will believe you. You can find all the good times you could ever imagine, along with any number of used condoms, tampons, old underwear, a solid log in the middle of the floor. Avoid at all cost. If possible, you are better off going into the woods and using whatever big leaves you can find.