Just read through the site. The "about us" button makes it appear that thousand trails and encore are setting a small number of sites aside for this membership promotion. There didnt seem to be anything guaranteeing any site at any location, and they didnt say that if the set aside sites were in use you would be able to get a different site.
As you said, they didn't seem to want to tell you much about the promotion. If you talk on the phone or visit a site, you will no doubt be given a long list of possible visits and stays, but they seem to be avoiding putting much in writing.
