Passport America is very reasonable. A couple nights will pay for it.
Other memberships can be very expensive if you don't travel in those areas and use it constantly. There are some folks who travel up and down the Pacific coast and stay in one of the membership parks. That will pay for it. But quite often, there is no park where you want to go if you travel in different states, especially the Midwest.
Also, how do you want to RV? Will you go to places just because you have a membership. Do you really want to see the U.S. and possibly won't find any membership parks to use?