We just financed a 2007 Safari. We used our credit union because they had an extremely low rate. However, we had to do two loans (one was a small personal to make up the difference) to get it fully financed and the reason is because of what many banks, credit companies, etc...use for valuation sources.
As you are probably well aware, financing a brand new Airstream is simple if you have good credit. Used ones are entirely different story. I say try your credit union first AND help them out with some comparables of recent past Airstream sales with your make and model. Google this and you will find them. This is what I did and it helped. We also just explained much about NADA and extremely inaccurate reporting or RV and Trailer sales to the advantage of dealers and how it is simply not a reliable source. In addition, we explained how airstreams are built and that the depreciation is marginal when compared with other brands of same size.
We only have financing for 48 months, so our monthly payment is high, but the interest rate is low and we will have it paid off swiftly.
If the low monthly payment is what you're after, you can try other sources like Good Sam's and Essex, but you may have to have a good size downpayment and longer payment period. You will also have a higher interest rate.