We lost our sweet Polly, our camping companion (dog), last Christmas. Gnocchi, our new kitty came to us that October, and has taken over the camping companion status from Polly. I never noticed ...
We don't travel with a cat, but my guess is that there would be no fee at most campgrounds. I'm much more curious (and honest) than I am a tightwad, so I'd probably always mention it just to see if the charge was applied or not.