We used the pop-up for a while -- consider a carpet for the floor though. The flooring is "crinkly" for lack of a better word. Freaked our cats out.
The stake idea is okay if your cat doesn't mind the harness. One of our cats would lay down and refuse to move when we put him on the stake.
We have a Carefree screen room attached to our rig.....however, it's not good for when you travel every few days.....it wears us out to put up and take down. :)
And you're right.....some RV places won't let you put up the pop-up.....but if you boondock a majority of the time, the cats can just stay in at those places.
Some years ago, someone posted a picture of their "cat cage" that hung out a window.....I've tried to find that again but can't find the link. It was perfect for the cats to get some fresh air. Our rig then had windows that didn't allow the cage (they were crank up windows)....but now we could use it. Hoping someone comes on here with the link. :)