Unless you are a full member, you have to use their "encore" area. Full members have access to the best sites in the better sections of the CG which have better views, are quieter, etc. We just stayed at a TT and we have associate membership only (we have full membership in another RV resort network). We had to stay in the encore area which was next to the main road but fortunately wasn't too bad. It only had full hookups at a few of the sites and some of them were shut down for maintenance on the electrical system.
You will get the pick of any available site in an encore area only, however, if a TT CG is not that busy they may give you the option of any site, but it's entirely up to the particular CG. We've had a couple of them like that.
Some TT campgrounds are outstanding like the one at Pacific City in Or. Others look tired and show their age and are in need of repairs and upgrades. But then, there's lots of others out there that are the same or worse. I like the rustic flavor they all seem to have, at least the ones we've seen.
I realy don't think we'd be members. I would recommend something like Coast to Coast or RPI. TT membership is broken down into zones so you have to pay more if you want to travel farther away plus their CG coverage in some parts of the country is poor or non-existent.
Thousand Trails zones & campgroundsThere are many other RV campground networks around. I would do your research and see how their locations and features, rules, benefits and costs compare before buying in. Sometimes you can buy a pre-owned membership for a lot less too but watch the restrictions and fine print.