I can't speak to in US cities (where indeed restaurants tend to police non-customer restroom use more regularly) but I've never had an issue with using a McDonald's restroom when I've been driving someplace in the US.
This is not just applicable to using the restroom as a non-paying customer. You can pretty much go anywhere if you just don't look guilty of doing something your not meant to be doing. They also say carrying a clipboard and looking in a hurry increases your "getting away" with it, whatever "it" may be.