I agree with the gas card.
As they are already RVers, they are going to have all the "stuff" they need (chairs, kitchen gadgets, camping gear, etc.).
A gift card to a specific store/place (Camping World, KOA, etc.) is only good if you KNOW they like that store/place. If you don't know for sure, then you risk giving them a card they will never use.
A gas card (or pre-paid credit card) that can be used at any gas station will be the most useful gift - they MUST get gas in order to travel.
The second choice would be a gift card to a restaurant - I'd get one that will give them a choice of restaurants rather than just one in particular.