Bottom line is there is no free (or discounted) lunch. Passport America is great for the 1 to 2 night stopover while going to a destination. We are members. Good Sam will get you 10%, not a big deal in my opinion. We are members.
Coast to Coast you are required to join a membership campground, the price of which can be quite high. We are members of C2C and we do use it but again, participating parks may not be anywhere near where you wish to go visit. C2C parks tend to be resort parks and have more amenities than the usual RV park with planned activities, snack bar/restaurant, putt putt golf and nice indoor and outdoor swimming pools. They will have more children as they are marketed to families.
In our travels we take advantage of what we come across when we come across it. I research all available options before hand of course. We never make reservations for just over night stays but always need and make them for our destination. It is what it is. In general, businesses are not in busines to give their product away. I don't begrudge them that.