Good Sam has absolutely no say in the policies of any "Good Sam" affiliated park. If the park's policy is Good Sam discount must be applied for at time of reservation, that is their policy. Marcus Lemonis nor any other employee of Good Sam Enterprises can force the park to change them.
Curious if you got another discount applied to your reservation such as Senior, Military, multi-day stay etc. If so, almost all parks will not stack discounts on top of each other so the the issue would be moot anyway.