Someone above mentioned RPI
Resort Parks International (RPI), Coast to Coast and Adventure Outdoor Resorts (AOR) are reciprocal programs that offer $9 or $10 per night camping at affiliated resorts. You have to be a member of a home resort. In some of the programs that can be almost nothing, in others it can be thousands of dollars per year. The advertised cheap camping sites at other resorts are limited. The affiliated resorts are for their own members first, and only left over sites are available for traveling folks.
They can be a good value / addon to a TT membership (RPI) but a person should investigate the individual program and how it might help their own usage/ travels before considering the additional costs.
Affiliated resorts/ campgrounds:
AOR - 140+
C2C - 400+
RPI - 285+
Every person's needs/ desires is different. TT/ Encore/ RPI are all valid and valuable options for many people. And not for others.