The problem with banks and credit unions is that they need collateral. While you many find a trailer you like for 10K, it will prob be more than 4 yrs old. Therefore the 'book' value on it may be much lower, say 5K. The banks will not give you 10K for a trailer that in Their books is only worth 5.
It is far easier to get financing for a newer vehicle/trailer, because if you default, the bank can recoup their money easily.
I know this because I went through the ringer buying my 'new' truck, an '05 chev silverado. No one wanted to finance me, because the vehicle was 'too old'. The price was 15k, but the book value was 7K.
That doesn't mean I could buy it for that price, that is basically the repo price should I default. Not good enough.
They wanted me to buy at least an '07, so it could be its own collateral.
My monthly payments would have been the same, but I would have been paying for 2 years longer, and almost 10K more.
I held out for the '05, but had to go through the dealership and got a poor percentage rate.
It is not easy to get financing on an older model.
Perhaps if you own your home, they may take that into consideration as collateral, with a cosigner.