I'm very, very new, in fact, I'm a moron when it comes to camping, campers, etc. I've been camping many times with friends and love it, but never have owned one. My friend has offered me her camper ...
Agreeing with many others - plywood (if that will be too rough, you can cover it with some inexpensive vinyl-tile flooring; that's what we did when our old popup's floor went soft. If it's free, enjoy it until it falls apart or until it develops in you either a distaste for RVing or (more likely) the desire for a newer rig. Good idea to have it parked somewhere (preferably with hookups) for a while and get used to using it.