I wouldn't use an empty site for dumping any more than I'd stop at a hotel and use an unlocked, empty room for a quick bathroom break without asking. In past times, I have asked the CG host and offered money for it, and often, they would either just allow me to dump at the FHU gratis, or just take a $5.
I do know a CG owner in north Texas ended up using a locking, removable Trafficguard bollard system because he had people driving in and dumping at FHU sites without paying at all hours of the night, especially in the winter around hunting season. He tried using padlocks on the lids on the campsites that were not occupied, but people wrenched them off. After he went to the locking barriers on campsites not in use, the problem pretty much ceased.
