I don't know what the RV rentals cost. I have heard numbers like $900 a week. That said, that's about $130/day plus about $75/day for a site, that's about $205 (ish) for a campsite and RV rental. Most cabins, I think tend to run upwards of $300, so as Denise said, it still could be cheaper then the cabins. Of course this is "Promotions" notwithstanding.
Either way, the Fort is awesome. Whatever you decide, I hope you have a great trip.