You can get financing on a 9 year old RV, and it will be limited to 6 years. Once it hits 10 years, most banks will not even loan on it. Smaller banks will with a much larger down payment, and good credit.
It always depends on the individual. With a personal guarantee, some people can borrow a lot of money unsecured. That is how financing an older RV would work.
The reason I sell my RV when it reaches 5-6 years of age is that the pool of potential buyers is much larger, allowing for a higher sales price. Most buyers with good credit will be able to purchase the RV in that age with little cash down. The pool of eligbile buyers gradually declines over the next few years.