I showed the pictures of the ramp to a friend at work who has had several RV's and he didn't think I would have a bit of trouble launching with a Seneca or any other rig. He thought my idea about changing to a really low drop hitch once I got into the park would help a lot too.
I just hate the idea of buying the rig and getting all the way down there and finding out I've got a problem. I still think it would be wise to rent a C with a big overhang and see if I have any issues launching with it. There are a Cruise America RV rental location near the park entrance.