We have memberships with Xscapers (Escapees), Good Sam and KOA. They pay for themselves fairly quickly after a few nights. However, weekly and longer rates are almost always lower than the discounted nightly rate. But sometimes a combination of weekly and nightly can be used when not staying an exact multiple of 7 nights.
We've looked into Thousand Trails, but it seems you have to use them a LOT for it to make sense and we don't want to base our travel around Thousand Trails locations.