I hesitate to suggest this, as most think it is a very bad idea....but if you have built up good equity in your house, you might look at a refinance of that to give you the money to buy the RV with cash. In our case, two years ago, we had 6 yrs left on the mortgage, at 4.8%. Doing a refi got us our fifth wheel and an 8 year mortgage at 2% (!!!) So monthly house payments only went up a little, and it's not very much longer till the payoff of both.
Current interest rates aren't that good, but they are still very low. It would at least be worth the time to crunch the numbers to see if you can make it work out to avoid the long repayment period that quickly puts you massively upside down in your value and equates to huge amounts of interest payments.