We've been members for a while and have stayed at some great places. Although the web page for the hosts only show the number of spaces for class A, B and C, I can't imagine any host turning away a TC. You call the host to inquire about availability anyway, so you can ask then. These sites are usually available on short notice. We've called the day before and have always gotten a site.
A word of warning: If you're looking at HH to save money (the sites are free to members), don't pick a winery. I've spent way more for a free site at winery than I even would for a site at a campground :)