Passport America has always paid for itself each year for us. You are correct that the availability of half price sites are limited. Each campground sets their own limitations. Some even don't offer half price sites at all during their peak season. We also look for city, county and state parks, Corps of Engineers campgrounds, state parks and KOA's. KOA has 10% off with their card and you earn points for discounted or free camping.
We choose 2 or 3 campgrounds in a location and compare the amenities to the cost per night. We also use weekly and monthly rates to save money.
we use www.woodalls.com to search for campgrounds and then check them on www.rvparkreviews.com