Look over the campground list which is online, and see if any of the campgrounds are someplace you go, or are on your routes. Check reviews of the potential campgrounds. If you have a firm base of usable campgrounds (at least enough savings to pay the yearly membership), it doesn't matter much if you hit others on your travels or not. Here in southern CA Pechanga and Riverside County Parks are on the list. I go to those enough to pay for the membership. On the road, I'll check if my route goes by any other PA campgrounds-then check the reviews of them. Some middling rated ones are good enough for an overnight stay. But like others said with driving costs it doesn't pay to drive much out of the way for a bargain campground.
Some of the PA campgrounds are very nice first tier campgrounds. Some aren't. They can't be broad brushed in this regard. I have never had problems getting the discount listed as long as I followed the campground notes (offering dates the discount valid). No grumpy/reluctant personnel due to Passport customer.