The gas station bathroom pics are the ones I cringe at having to do. It's always a gamble as to how clean they are and I've had to walk right out of a few of them. It's hard not looking weird in a bathroom that has multiple stalls because you have to dodge people and strange looks. The only thing we have working in our favor nowadays is that nearly everyone always has their phones out and are taking pictures, etc. If I'm alone, I try to get a panoramic pic so the whole bathroom is included. If I don't have that kind of time, I take multiples, especially if I see anything that's an issue, and then do a collage in Publisher and save it as a JPEG to upload. I'd rather them kick it back to me and say I need to only upload one pic instead of not having what they want and that being the only pic I have.
As for the legality, just be super careful to not get people in the shots. Nowadays everyone is so litigious and it's just better the play it safe, even if it adds time on to the shop. I Googled your original concern and it seems that in some states taking pictures in a bathroom could be concerned "obscene," so who knows.