I've never been in that situation being a TT and 5er consumer, but we always carry a small tent of some kind with us that can be set up quickly. Right now, we have a port-a-potty / shower pop-up wire type tent thingie that folds up flat in a bag. We've used to carry a kiddie pop-up tent with us, all the time, but never used it for marking an empty campsite. But it was there for the grandkids to play in.
FYI, I love that shower-pop-up-port-a-potty tent thing, because I really like taking showers outside. No... I don't do that at campsites, I do it at home!
