We have had it for the past 10 years and have over 1,000 nights in the system. At $30-60/night, we’ve saved $20-50,000 in that time.
Basically it’s a camping membership. You either pay a yearly fee for the base membership or a one-time fee + yearly fees for an upgraded membership. It’s essentially pre-paying for campsite fees. The more you use it, the better the deal is. The biggest thing is that there are 2-3 week stay limits. There are about 80 parks in the Thousand Trails system plus another 115 in a membership add-on. Once you pay, the parks are then $0/night.
It can be a huge savings if there are parks where you want to be. For instance, we bounce around Florida all winter and generally don’t pay anything unless we choose to stay out of the system.
The other benefit is that it is very popular with families who fulltime. We come across lots and lots of our friends in the system.