Probably the cheapest way to finance and RV is to establish a Home Equity Line of Credit (HELOC) against equity in a stick house. Draw out the money from the HELOC and then pay cash for the RV.
The HELOC doesn't care if you're buying a used or new RV or any other thing. The interest rate on our HELOC is 2.7%. We just leave this line of credit open (no charge for keeping it open) continuously so we can draw out cash whenever we need to for large emergencies or large purchases. Any amount we owe the HELOC we can pay back at any speed above the minimum monthly payment speed.
IMHO, a HELOC is an excellent way to get cash conveniently for anything anytime with no questions asked other than "how much do you want to borrow?". It can all be done from your computer too.