Aside from the lecturing about not buying toys on time, the OP has been given some good advice here. I'm of the old school of paying cash for toys too but that is not the topic here.
Step 1: Develop a realistic estimate of the difference between what you owe on the coach and what it is worth. Bear in mind you are at the end of the strongest selling season now. OTOH it is clear the retiring baby boom demographic is helping to strengthen RV sales too.
Step 2: Contact the lender about selling the coach and continuing to make payments until the loan is paid off. This may get you nowhere if you borrowed from one of these predatory big banks like Wells Fargo, Chase or BOA. But if you have a great credit history, maybe your lender will see the wisdom of allowing you to continue to pay down the principal after the coach is sold.
Step 3: If you have another asset like a house, maybe you can leverage that to pay off the RV loan. Even second loans are still cheap by historic standards.
Remember you are also getting rid of the expense of maintaining, driving and parking the coach. That is money you can use to get this loan off your back.
Do not wreck your credit by walking away. They will do exactly as described--wholesale the coach out for a lot less than you could get for it and then come after you for the difference and whatever crap they can pile on top.
It would help folks here to offer good advice if you could indicate just how far underwater you think you are with this coach. Be conservative with the resale value however, it appears there are a lot of private sellers out there with really unrealistic ideas about what the market will bear.