I commend you for being conscientious and paying your bills. As I see it you have two options.
As described above, take out a personal loan for the amount of negative equity and sell the RV. You will then save the storage, insurance, and other costs so your expenses might actually be lower.
Option two I would not recommend but it is possible. If you buy a smaller RV and roll the negative equity into it you can also accomplish your goal. Problem is that you still owe the same amount and now have a smaller RV that you soon won't use either. I don't know you but from the information given, it sounds like your RVing days are over. Rolling the debt into a smaller RV ultimately puts you in a worse position down the road a year or so.