Good Sam pays nothing to the parks. It is the other way around, parks pay Good Sam to be affiliated for advertising purposes and they agree to offer the 10% discount to Good Sam Members. If Good Sam mandated a 25% discount those parks would either just cease being Good Sam Affiliated parks, which would be bad for Good Sam, or just raise their prices to the point they would still be getting the same amount of money.
Not sure what gas prices have to do with it, since the Oil Companies don't own many RV parks and Good Sam doesn't have too many gas stations or oil wells.