Al and Nita,
No problem having an opinion. I may have been a little harsh responding about the math. Sorry.
Also, everyone, remember this. The campgrounds allocate a percentage of their sites to discounts. Once that allocation has been met it may be that you will get a "no vacancy" when you stop unannounced, but you see many sites vacant. It's just the way the CG's operate. That is why many of the PA CG's state to call ahead for availability.
The PA CG Book is a good reference, but the newest and latest updates are on their web site. You need to read the fine print. Some only honor 1 night PA, some all nights, and some just certain nights. You do have to plan ahead. As with all CG's, not all are the cream of the crop, and you need to take your lumps with your cream. All in all, we have found it to be of benefit to us in our travels, and we will continue to use it, yearly or lifetime - don't know yet.