Both Good Sam and Passport America are membership organizations and only member campgrounds give the discount. There are GS member campgrounds located all over the country and are plentiful. The discount is 10% off the regular rate.
The Passport America is supposed to be 1/2 off the regular rate but does not seem to always work out that way. In addition, there are not nearly as many PA member campgrounds as there are Good Sam parks.
Neither one, as far as I know, is good at any state, federal, county or city campgrounds, but there may well be exceptions to this.
We are members of GS and used to be members of both Coast to Coast and Passport America. Did not find the last two very useful where we travel. We do use Good Sam club campgrounds quite frequently though as there are so many of them located all over.
I would check out where the Passport America campgrounds are located and see if you would use them enough to justify a membership in that organization.
Hope this helps.
Barney