It's what we call portojohns It's the only thing I've ever heard it referred to as

Funny. I wasn't familiar with the porta potty term, only the phrase.

It might just be a brand, but I've seen so many porta potties with "Johnny on the Spot" on the side that it's all I can see/hear

Both a brand and a phrase used for non brand like the old copy machine xerox type thing but the phrase been around centuries. First time I saw the brand, yep STL. Lmao. They use to call a baseball player Johnny on the spot. Probably a few.