I wrote a couple of things that will hopefully help:
Thousand Trails Overview
Is Thousand Trails Right for you?
Longest story shortest- the parks are best described as "the price is right". The parks are usually not great and are more like somewhere between "meh" and "good". I'm not picky at all and am just looking for somewhere to hookup for 1-3 weeks. They work great for me.
In 2015, I had 65 TT nights- mostly in the western part of the country. That averaged $7/night with the cost of my membership and add-on costs (some parks charge for 50-amp service).
In 2016, I have had 35 or so and have another 65 planned through October at this point. I might actually end up with more as we don't know what our summer plans fully are yet. But my cost per night has gone up since I upgraded my membership to a higher level.
They really help me manage my overall cost per night. In 6-months of fulltiming in 2015, I ended up at $31/night. 2016 is all over the board as we've spent quite a bit of time at Disney's campground.
And lastly, the biggest thing to be aware of is that all of the parks are "first come, first served". You're not guaranteed a specific site, full hookups, or 50-amp service. Just that there is somewhere for you to park.
And, you can only make reservations 60/90/120 days ahead of time- depending on your membership contract.