I'd guess/assume that most new RV purchases are financed.
RV loans look to be good business for lenders:
Consumer Delinquencies Fall Slightly in Second Quarter
The being
upside down can be masked by a high trade-in value against an overpriced new unit.
If not trading in then it would likely be hard to avoid a hit if the down payment was very low and financing term very long and the sale was taking place early in the loan term.
I wouldn't buy a used RV without first doing a lien search. For me, that now includes used units purchased from a dealer. My last RV purchase was from a dealer and a lien search turned up a past lender who's lien hadn't been taken off the title. It added a few days to finalizing the purchase because I wouldn't purchase it without a clear title.
As for financing; a method here is a loan that is secured by the asset purchased - I joked with a dealer that basically all I needed was a "pulse" to qualify for a 20 year RV loan. Based on that experience I'd assume that it is pretty easy to purchase whatever your heart desires.