Never had that happen to me, and we've stayed in many Good Sam campgrounds. We were at the Good Sam campground in Custer, SD a couple weeks ago. They gave us the 8th night free, and the 10% Good Sam discount after that, and we paid with Credit Card. We then later paid for our Daughter and Son-in-laws campsite and got the same deal, and then we also rented one of their Tee-Pee sites for one night, and got the discount too, all with Credit Card.
We stayed at Good Sam campgrounds from Florida to Nebraska, and always got the discount with our credit card.
Someone from Good Sam needs to address the inconsistency. But, we've never had any issues.