I think you really only have two options here and that's keep paying or turn it in to the bank. The first thing you need to do is get in touch with somebody at the bank and explain the situation as see if they have any suggestions for you. The question you need to ask them is if you turn it in and they sell it, are they going to come after you for the difference. I'm pretty sure in most States when it comes to vehicle loans, they can come after you for the difference.
If they can come after you for the difference here's your problem: Right now you say your $50k upside down. I assume that is the difference between what you think you can sell it for and what you owe. If you do a voluntary repo and let the bank take it back they aren't going to sell it for retail. They are going to send it off to an auction company to sell it and they may end up selling it for $75k lower than the loan balance. That means instead of owing $50k you now owe $75k to the bank. Either way you can probably arrange payment options that may be cheaper than your current motorhome payments?
Then there's always bankruptcy too. That gets you out of the debt but puts a bigger black mark on your credit report than a repo would.