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If you buy from a dealer they will usually offer financing but you might want to check other resources. Usually the RV magazines contain adds for different finance institutions. Also don't forget to check with you bank.
Do a web search on "RV financing" (without the quotes) and you will get several hits. Generally you will do much better with a company that specializes in Rv financing than through a local bank or even a credit union. That's because most of the car financing outlets don't offer loan terms that fit a major expense like a big RV. RV financers offer home mortgage like terms, e.g. large loans for 15-20 years at fairly low rates. They can provide loans for either dealer or private sales.
Dealers get a hefty commission on the financing they sell, so it generally costs more to finance through a dealer. However, the dealer's financing deal is usually subject to negotiation and they will often match a lower rate if you tell them you have a better quote from elsewhere.
The Good Sam Club works with one of the major Rv financers [E-trade Financial] and they generally offer decent terms. You might check there first. http://www.goodsamclub.com/benefits/index2.cfm?action=category&Cat_ID=7