There is no legitimate financial reason to buy new. You are wasting money on something that will depreciate almost the second you drive it off the lot.
That said, there is a huge difference in what an RV looks like new today vs even a few years ago. Floor plans, larger slides, straight roof lines all contribute to a massive increase in how things feel on the interior.
So while there may not be a financial reason to buy a new unit, many people do because that is what they want.
Don't like to yourself though, your new unit will likely need just as much work as the older one.